We evaluated the frequency of antibodies to glutamic acid decarboxylase (GAD-Ab) in Japanese patients diagnosed initially as having non-insulin-dependent diabetes mellitus (NIDDM) and investigated a possible link between the presence of GAD-Ab and development of the insulin-dependent (ID) state. The population sample consisted of 583 Japanese NIDDM patients (age at onset > 30 years) who were initially non-ketotic and did not require insulin treatment during at least 6 months of observation. GAD-Ab were measured using radioimmunoassay. The clinical characteristics of GAD-Ab+ patients were carefully examined at four-year intervals from the onset of diabetes. We also examined the ID state by measuring the level of postprandial serum C-peptide and i.v. glucagon-stimulated serum C-peptide. The overall prevalence of GAD-Ab in Japanese NIDDM patients was 3.8%. The frequency of GAD-Ab+ did not significantly decrease with a long history of diabetes. GAD-Ab+ patients had a lower body mass index, compared with GAD-Ab- (20.8 +/- 2.9 vs 23.0 +/- 3.7, P < 0.005), lower postprandial C-peptide levels (0.7 +/- 0.6 vs 1.4 +/- 1.2, P < 0.01), and an early commencement of insulin therapy (3.6 +/- 4.7 vs 8.3 +/- 6.6, P < 0.01). GAD-Ab+ patients who had already developed the ID state had characteristically higher titers of GAD-Ab (421.4 +/- 359.1) and a higher frequency of islet cell antibodies (ICAs) (77.8%), compared with GAD-Ab+ NID patients (titer: 60.2 +/- 86.9, P < 0.005, 23.1%, P < 0.05, respectively). GAD-Ab+ ICAs+ patients showed higher frequencies of ID state at any diabetic duration compared with GAD- ICAs-, while GAD-Ab+ ICAs- patients did not differ in the frequency of the ID state from GAD- ICAs-. Our results suggest that the presence of both GAD-Ab and ICAs represents a high risk for IDDM in GAD-Ab+ NIDDM patients.