We report a 27-year-old man with insulin-dependent diabetes mellitus and transient-pituitary-isolated gonadotropin deficiency. He had typical diabetic symptoms, and loss of libido of a 6-month duration. Although antibodies to islet cells or islet cell surface were not detected in his sera, daily urinary excretion of c-peptide immunoreactivity was extremely low, and antibodies to the pituitary AtT-20 cell were detected. The plasma responses of gonadotropin to a single and a repetitive luteinizing hormone-releasing hormone were extremely low, whereas testosterone concentrations in the serum and urine were low normal. After 6 months, the gonadotropin deficiency and loss of libido were not detected and antibodies to the AtT-20 cell was negative. We suspected that both insulin-dependent diabetes mellitus and transient-gonadotropin-deficiency might be an autoimmune mechanism.