The aim of this study was to evaluate the insulin dynamics of patients with gestational diabetes mellitus (GDM) and to compare perinatal outcomes according to the insulin response patterns. Twenty-two out of 925 consecutive women examined were diagnosed as having GDM. One hundred and ten women who experienced a normal pregnancy were used as controls. Plasma glucose levels and insulin responses were evaluated by a 2-hour 75-gram oral glucose tolerance test (OGTT). Immunoreactive insulin (IRI), the area under the curve (AUC) of glucose (AUCg) and insulin (AUCi), and the insulinogenic index (II = DeltaIRI 30 min/DeltaBS 30 min) were measured. The GDM patients were divided into three subgroups, consisting of hyper-, normo- and hypoinsulinemic groups, according to the mean +/- 2 SD of the AUCi obtained from the controls. Clinical and laboratory findings were compared among the GDM subgroups and controls. The GDM patients showed impaired insulin secretion to glucose stimuli, with low plasma insulin levels (at 30 min) and reduced insulin/glucose ratios (at 30 and 60 min) early in the 75-gram OGTT. The II and AUCi/AUCg values of GDM patients were reduced as compared with those of controls. These reduced insulin responses were remarkable in hypo- and normoinsulinemic GDM patients, but were not detected in hyperinsulinemic GDM patients. The number of babies large for their gestational age in normo- and hypoinsulinemic GDM patients was significantly higher than that in hyperinsulinemic GDM patients or controls. Hyperinsulinemic GDM patients had a high frequency of pregnancy-induced hypertension (40%). The body mass index prior to pregnancy of hyperinsulinemic GDM patients was significantly higher than that of normoinsulinemic GDM patients or controls. It was demonstrated that not only insulin secretion, but also perinatal clinical characteristics, differed among the GDM subgroups. The heterogeneity of the disease was thus confirmed.