Background: Cancer of unknown primary (CUP) has a poor prognosis. Given the recent approval of immune checkpoint inhibitors for several cancer types, we carried out a multicenter phase II study to assess the efficacy of nivolumab for patients with CUP., Patients and Methods: Patients with CUP who were previously treated with at least one line of systemic chemotherapy constituted the principal study population. Previously untreated patients with CUP were also enrolled for exploratory analysis. Nivolumab (240 mg/body) was administered every 2 weeks for up to 52 cycles. The primary endpoint was objective response rate in previously treated patients as determined by blinded independent central review according to RECIST version 1.1., Results: Fifty-six patients with CUP were enrolled in the trial. For the 45 previously treated patients, objective response rate was 22.2% [95% confidence interval (CI), 11.2% to 37.1%], with a median progression-free survival and overall survival of 4.0 months (95% CI, 1.9-5.8 months) and 15.9 months (95% CI, 8.4-21.5 months), respectively. Similar clinical benefits were also observed in the 11 previously untreated patients. Better clinical efficacy of nivolumab was apparent for tumors with a higher programmed death-ligand 1 expression level, for those with a higher tumor mutation burden, and for microsatellite instability-high tumors. In contrast, no differences in efficacy were apparent between tumor subgroups based on estimated tissue of origin. Adverse events were consistent with the known safety profile of nivolumab. No treatment-related death was observed., Conclusions: Our results demonstrate a clinical benefit of nivolumab for patients with CUP, suggesting that nivolumab is a potential additional therapeutic option for CUP., Competing Interests: Disclosure JT: reports personal fees from AstraZeneca K.K., Boehringer-Ingelheim Japan Inc., Bristol Myers Squibb Co. Ltd., Chugai Pharmaceutical Co. Ltd., Eli Lilly Japan K.K., Merck Sharp & Dohme (MSD) K.K., Taiho Pharmaceutical Co. Ltd, outside the submitted work. KY: reports personal fees from AstraZeneca K.K., Chugai Pharmaceutical Co. Ltd, Daiichi Sankyo, Eizai, Eli Lilly, Fuji Film Pharma, Genmab K.K., Novartis Pharmaceuticals K.K., Ono Pharmaceutical Co. Ltd, Pfizer Japan Inc., Taiho, Pharmaceutical, Takeda Pharmaceutical Company Limited, outside the submitted work. Grant from AstraZeneca K.K., Boehringer Ingelheim, Chugai Pharmaceutical Co. Ltd, Daiichi Sankyo, Eizai, Eli Lilly, Kyowa-Kirin, Nihon Kayaku, MSD, Novartis Pharmaceuticals K.K., Oncotherapy Science, Ono Pharmaceutical Co. Ltd, Pfizer Japan Inc., Sanofi, Seagen, Taiho Pharmaceutical, Takeda Pharmaceutical Company Limited, outside the submitted work. HM: reports grants, personal fees and other from Bristol Myers Squibb, Ono Yakuhin, during the conduct of the study; personal fees from Abbvie, grants from Asahi-Kasei Pharma, grants from Astellas, other from AstraZeneca, grants, personal fees, and other from Bayer, grants and personal fees from Boehringer, grants, personal fees, and other from Bristol Myers Squibb, personal fees from Celgene, grants, personal fees, and other from Chugai, grants from CSL Behring, grants, personal fees, and other from Daiichi Sankyo, grants and personal fees from DaiNihonSumitomo, grants and personal fees from Eizai, grants, personal fees from Genomic Health, personal fees from Janssen, grants and personal fees from Kyowa-Kirin, grants and personal fees from Lilly, grants and personal fees from Merck Serono, grants, personal fees, and other from MSD, grants from Nihon Kayaku, grants from Nihon Shinyaku, personal fees and other from Novartis, personal fees and other from Ono Yakuhin, personal fees from Otsuka, grants, personal fees, and other from Pfizer, grants and personal fees from Sanofi, personal fees from Shire Japan, grants from Shionogi, grants, personal fees, and other from Taiho, grants and personal fees from Takeda, grants from Teijin Pharma, grants from Yakult, outside the submitted work. YT: reports grants, personal fees, and other from Bristol Myers Squibb, Ono Pharmaceutical Co. Ltd, during the conduct of the study; personal fees from AstraZeneca, grants and personal fees from Boehringer Ingelheim, grants and personal fees from Chugai Pharmaceutical Co. Ltd, grants from Daiichi Sankyo, grants and personal fees from Eli Lilly, grants from Kyowa-Hakko Kirin, grants and personal fees from MSD, grants and personal fees from Novartis, grants and personal fees from Taiho Pharmaceutical Co. Ltd, grants from Takeda, outside the submitted work. YM: reports personal fees from Ono Pharmaceutical, Bristol Myers Squibb, MSD, and Chugai, research funding from MSD, outside the submitted work. YS: reports grants from Kindai University, during the conduct of the study; grants from Eisai Co., Ltd, personal fees from Japanese Society of Medical Oncology, grants from Novartis Pharma K.K., grants and personal fees from Taiho Pharmaceutical Co., Ltd, outside the submitted work. ST: reports grants from Ono, during the conduct of the study; personal fees from Asahi Kasei, personal fees from Bayer, personal fees from Chugai, personal fees from Japan blood products organization, personal fees from Medicon, grants and personal fees from Merck Biopharma, personal fees from Nippon Kayaku, personal fees from Taiho, personal fees from Yakult, outside the submitted work. YC: reports personal fees from Chugai Pharmaceutical Co., Ltd, outside the submitted work. KS: reports personal fees from Roche Diagnostics, Bio-Rad, AstraZeneca, Chugai Pharmaceutical, and Hitachi outside the submitted work. KN: received grants and personal fees from Eli Lilly, Nippon Boehringer Ingelheim, grants from Ignyta, Korea Otsuka Pharmaceutical, Thoracic Oncology Research Group, North East Japan Study Group, and personal fees from Chugai Pharmaceutical, Eisai, Pfizer, Novartis Pharma, MSD, Ono Pharmaceutical, Bristol Myers Squibb, SymBio Pharmaceuticals, Life Technologies Japan, Solasia Pharma, Yakult Honsha, Roche Diagnostics, AstraZeneca, Otsuka Pharmaceutical, Sanofi, Guardant Health, Amgen, outside the submitted work. KN: reports grants and personal fees from AstraZeneca K.K., grants and personal fees from Chugai Pharmaceutical Co., Ltd, grants from Daiichi Sankyo Co., Ltd, personal fees and other from Eli Lilly Japan K.K., grants and personal fees from Merck Biopharma Co., Ltd, grants and personal fees from MSD K.K., grants and personal fees from Nippon Boehringer Ingelheim Co., Ltd, personal fees and other from Ono Pharmaceutical Co., Ltd, grants, personal fees, and other from Pfizer Japan Inc., personal fees from Care Net, Inc., personal fees from 3H Clinical Trial Inc., grants from EPS International Co., Ltd, grants from EPS Corporation, grants from Kissei Pharmaceutical Co., Ltd, personal fees from Kyowa Kirin Co., Ltd, personal fees from Nippon Kayaku Co., Ltd, grants from Parexel International Corp., grants from IQVIA Services Japan K.K., grants from SymBio Pharmaceuticals Limited, grants from Syneos Health, personal fees and grants from Takeda Pharmaceutical Co., Ltd, grants from Taiho Pharmaceutical Co., Ltd, personal fees from Medical Review Co., Ltd, personal fees from Medical Mobile Communications co., Ltd, personal fees from Nikkei Business Publications, Inc., personal fees from Amgen Inc., grants from Eisai Co., Ltd, grants from PRA Health Sciences, grants from Mochida Pharmaceutical Co., Ltd, grants from Covance Japan Inc., grants from Japan Clinical Research Operations, grants from GlaxoSmithKline K.K., grants from MSD K.K., grants from Sanofi K.K., grants from PPD-SNBL K.K, grants from SymBio Pharmaceuticals Limited, grants from Sysmex Corporation, grants from Medical Research Support, personal fees from Yodosha Co., Ltd, personal fees from Yomiuri Telecasting Corporation, personal fees from 3H Clinical Trial Inc., outside the submitted work. HH: reports grants from AstraZeneca K.K., Boehringer Ingelheim Japan Inc., Chugai Pharmaceutical Co. Ltd, and Ono Pharmaceutical Co. Ltd, personal fees from AstraZeneca K.K., Boehringer Ingelheim Japan Inc., Bristol Myers Squibb Co. Ltd, Chugai Pharmaceutical Co. Ltd, Eli Lilly Japan K.K., Kyorin Pharmaceutical Co. Ltd, MSD K.K., Novartis Pharmaceuticals K.K., Ono Pharmaceutical Co. Ltd, Pfizer Japan Inc., Shanghai Haihe Biopharm, Taiho Pharmaceutical Co. Ltd outside the submitted work. All other authors have declared no conflicts of interest., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)