The frequency of insulin-dependent diabetes mellitus in the Turkish adult-onset diabetic population has not been assessed previously. In the present study, we retrospectively evaluated the medical records of 801 Turkish patients with adult-onset (> or = 30 years) diabetes to determine the frequency of cases diagnosed as insulin-dependent diabetes. Fifty-two (6.5%) patients met our criteria of adult-onset insulin-dependent diabetes mellitus. At disease onset, 20 patients presented with ketoacidosis (38.5%), while 32 patients (61.5%) were non-ketotic. In the insulin-dependent diabetic group, islet cell antibodies were positive in 10 out of 16 (62.5%) patients studied. In contrast, none of the 16 patients had positive reactions with respect to insulin autoantibodies. Twelve out of 20 patients (60%) had glucagon-stimulated C-peptide levels above 0.6 nmol/l, suggesting a sufficient insulin secretory reserve. In view of these observations, we conclude that insulin-dependent diabetes mellitus is not rare among patients with adult-onset diabetes in the Turkish population. In a majority of cases, the disease onset is non-ketotic. Beta-cell function is relatively preserved, and insulin autoantibodies do not develop at diagnosis. In contrast, islet cell antibodies are frequently present at the onset of clinical insulin-dependent diabetes, possibly indicating continuing beta-cell destruction.