A 29-year-old man was observed to develop insulin-dependent diabetes mellitus following a 5-month treatment with recombinant α-2b-interferon for chronic hepatitis C. After the onset of the disease, serum samples that had, respectively, been collected before therapy commencement, at month 3, and at the onset of insulin-dependent diabetes mellitus were tested for islet-cell (ICA-IgG), glutamic acid decarboxylase (GAD-Abs), IA2 (IA2-Abs) and insulin (IA-Abs) autoantibodies. The following results were obtained: ICA-IgG, 5, > 80, and > 80 JDF-U, respectively; GAD-Abs: > 100 U/ml in all three measurements; IA2-Abs and IA-Abs: negative. During treatment, thyroid microsomal autoantibodies increased markedly (from 1:100 to 25 600 titer); thyroid-stimulating hormone was persistently normal. HLA class II typing revealed a genetic predisposition to insulin-dependent diabetes mellitus as demonstrated by the presence of DRBI* 04 08 , DQ A1 52 Arg+/Arg+, and DQB1 57 N-Asp/Asp alleles. One year after the onset of insulin-dependent diabetes mellitus, the patient is still receiving 30 IU insulin daily; the liver function tests are normal and HCV-RNA is negative. These data support the hypothesis that, in predisposed patients, α-interferon therapy can enhance an ongoing autoimmune process against pancreatic β-cells and induce overt insulin-dependent diabetes mellitus. We therefore suggest that, in patients with a documented predisposition to insulin-dependent diabetes mellitus, α-IFN therapy should be administered with caution.