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Antibodies targeting the receptor programmed death 1 on T cells have been approved for the treatment of lung cancer. Immune checkpoint inhibitors (ICIs) induce various immune-related adverse events. Life-threatening hematotoxicity can be provoked by ICI therapy. Although ICI-related endocrinopathy and interstitial lung disease have been well documented, hematotoxicity requiring intensive treatment is relatively rare. We describe a case of nivolumab induced thrombocytopenia after transient mild fever. A 77-year-old man with non-small cell lung cancer was administered nivolumab (240 mg/body, every 2 weeks) as second line therapy. On the day 2 after the first nivolumab infusion, he had a fever and his C-reactive protein level was elevated. Thoracic computed tomography revealed no interstitial lung disease or pneumonia. The fever resolved on day 9 and was not seen thereafter. On day 15 after the first nivolumab infusion, severe thrombocytopenia suddenly emerged. A bone marrow examination revealed no dysplasia or invasion. Based on the presence of high platelet-associated IgG titer, normal bone marrow plasticity and a lack of effectiveness of platelet infusion, we diagnosed nivolumab-induced immune thrombocytopenia. Daily administration of 60 mg of prednisolone restored the patient's platelet count and platelet-associated IgG. We also found that there was significant shrinkage of the primary lesion and that stable disease was achieved. One must be aware of this relatively rare side effect and the unusual clinical findings that could be associated with immunoreaction. Keywords: Immune checkpoint inhibitors, Nivolumab, Thrombocytopenia, Lung cancer, Immune-related adverse events, Platelet-associated IgG