Cancer of unknown primary: ESMO Clinical Practice Guideline for diagnosis, treatment and follow-up.

Competing Interests: Disclosure AK reports personal fees as an invited speaker from Roche; fees paid to his institution for advisory board membership from Roche; institutional funding from Bristol Myers Squibb (BMS); non-remunerated role as a principal investigator for Roche. TB reports fees paid to his institution as a study oncologist, for expert testimony and study-related travel expenses from Roche. CP reports fees paid to her institution as an invited speaker from Roche; institutional funding as a coordinating principal investigator from Roche. GB reports personal fees for advisory board membership from Gensenta; institutional funding as a Steering Committee member from Bayer; non-remunerated roles as a principal investigator for Eli Lilly, Merck Sharp & Dohme (MSD) and Roche; member of the American Society of Clinical Oncology (ASCO). SD reports personal fees as an invited speaker from Bracco Germany; non-remunerated leadership role as President (2010-2012) and Vice-President (2012-2014) of DEGUM (German Ultrasound Society); non-remunerated role as member and chairman of the ‘Radiation Protection in Medicine’ (2017-2020) working group for the Radiation Protection Commission (SSK) at the German Federal Ministry of Environmental Protection. KH has reported no potential conflicts of interest. LM reports a non-remunerated role as co-chair of the Steering Committee for the CUPISCO trial from Roche. HM reports personal fees as an invited speaker from Amgen and Roche; personal fees for advisory board membership from AstraZeneca, Janssen and Merck; institutional funding from Roche. KO reports institutional funding from BioClavis, BioTheranostics and Leica; non-remunerated advisory role as member of Steering Group for Early Detection and Diagnosis of Cancer Roadmap for Cancer Research UK and as member of Innovation Expert Advisory Group (EAG) for NHS England Cancer Innovation Programme; co-author of the RCPath dataset on CUP and malignancy of unknown origin for the Royal College of Pathologists; Cancer Research UK affiliate at the Beatson Institute; member of the National Cancer Research Institute (UK). AP reports personal fees for a congress subscription from Amgen; personal fees for advisory board membership in urothelial cancer from Basilea; personal fees for congress expenses from Janssen. HW reports personal fees as an invited speaker and for advisory board membership from Array BioPharma, Bayer, BMS, Celgene, Erytech, Incyte, Merck KGaA, Pierre Fabre, Servier Sirtex Medical and Roche/Genentech; personal fees for consultancy from OnoSil; personal fees for an advisory role and as a Trial Steering Committee member from Zymeworks; personal fees and institutional funding for an advisory role, as a coordinating principal investigator and member of Trial Steering Committee from Sirtex Medical; non-remunerated advisory roles with Bayer, Pfizer and Pierre Fabre. GZ reports personal fees as an invited speaker from Amgen, Ipsen, Leo Pharma and Merck. GP is the Chief Medical Officer for ESMO and member of ASCO and the Hellenic Cooperative Oncology Group and the Hellenic Society of Medical Oncology (HeCOG). KF reports personal fees for advisory board membership from Curevac and Orion; fees paid to his institution as an invited speaker from Astellas, AstraZeneca, Bayer, Janssen, MSD, Novartis, Pfizer and Sanofi; fees paid to his institution for advisory board membership from AAA, Astellas, AstraZeneca, Bayer, Janssen, MSD, Novartis/AAA and Pfizer; institutional funding as trial chair from AstraZeneca, Bayer, BMS, Janssen, MSD, Orion and Pfizer; non-remunerated role as principal investigator and trial chair for Bayer, BMS, Merck, Novartis/AAA and Orion. TO has declared no conflicts of interest.