Abstract
A 73-year-old man who was a current smoker complained of weakness in his limbs and slow movement and was diagnosed with primary lung melanoma with brain metastases. Following stereotactic brain radiotherapy, nivolumab was administrated. After the first cycle of nivolumab, his blood neutrophil count and hemoglobin levels started to decline. Excluding other possible causes, nivolumab was considered the most probable cause of bicytopenia. Nivolumab was not restarted, and the bicytopenia gradually recovered with no corticosteroid administration for this event. While serious hematological adverse events regarding immune checkpoint inhibitors have been assumed to be rare, severe neutropenia and anemia should be considered in patients receiving immune checkpoint therapy.