A 61-year-old man was observed to develop type 1 diabetes mellitus following a 3-month treatment with recombinant alpha-2b peginterferon combined with ribavirin for chronic hepatitis C. Serum samples, collected before the start of therapy and 2 months after the diagnosis of diabetes mellitus, revealed islet-cell antibodies at a titer of 20 and 40 JDF-U, respectively, and glutamic acid decarboxylase autoantibodies at a value of 76.5 and 196 IU/ml, respectively. Antibodies to second islet autoantigen were persistently negative. HLA class II typing revealed the presence of DRB1*04/DRB1*14, DQA1*0303-0104 and DQB1*04-0503 alleles. Eight months after the onset of type 1 diabetes mellitus, the patient is still receiving 30 IU insulin daily; the liver function tests are normal and serum hepatitis C virus RNA is negative. These data confirm that, in patients with potential diabetes mellitus, the disease may become manifest as a side-effect during therapy with peginterferon-alpha plus ribavirin. The patient as a candidate for interferon treatment should therefore be investigated, in addition to thyroid autoimmunity, also for pancreatic autoantibodies before starting therapy.