To identify the primary pathogenic factors involved in the development of Type 2 diabetes mellitus (DM), we studied Japanese women with former gestational diabetes mellitus (GDM) who are at risk for the later development of Type 2 DM. We used the minimal model analysis derived from frequently sampled intravenous glucose tolerance test (FSIGT). The subjects consisted of eight non-obese women with a history of GDM and eight non-obese normal women as control subjects. The 75 g oral glucose tolerance test (75 g OGTT) performed within 6 months of delivery confirmed that all the subjects with former GDM had a normal glucose tolerance. Insulin sensitivity (SI) derived from the minimal model analysis was not different between the two groups. Glucose effectiveness at zero insulin (GEZI), reflecting tissue glucose sensitivity, was significantly lower in former GDM patients than in control subjects (1.18+/-0.34 vs 2.26+/-0.29 x 10(-2) min(-1), p < 0.05). The early phase insulin secretion found in FSIGT was markedly reduced to 56% of that observed in control subjects (1250+/-87.4 vs 2223+/-304.3 pmol l(-1) min, p < 0.01). Our results indicate that in former GDM patients, who are japanese and non-obese, impairment of the acute insulin response to glucose and a decrease in tissue glucose sensitivity rather than insulin sensitivity are the primary pathogenic factors involved.