Anti-programmed death-1 (anti-PD-1) monoclonal antibodies, such as nivolumab, have been used for the treatment of various types of cancers, and excellent efficacy has been shown in some patients. The adverse effects of anti-PD-1 antibodies relating to autoimmunity are different from traditional chemotherapeutic drugs and may involve many organs including the endocrine system. We herein describe a case of adrenocorticotropic hormone deficiency during the treatment of advanced small-cell lung cancer, probably caused by nivolumab-induced hypophysitis. The case showed nonspecific, insidious, as well as potentially life-threatening characters of immune-related adverse effects. It is important for physicians to acknowledge clinical features of the rare side effect and take appropriate and prompt treatment.