Gestational diabetes mellitus (GDM) is associated with much increased risk of developing diabetes later on in life. Using the frequently sampled intravenous glucose tolerance test and the minimal model analyses we have therefore determined the early insulin response to glucose (EIR) and insulin sensitivity (Si), in women with GDM of different severity (n = 14) and in normal women (n = 10). During the last trimester of pregnancy. GDMs compared to controls had significantly lower EIR (p < 0.001) and Si (p < 0.01). The reduction in EIR was less marked in GDM patients treated with diet alone (n = 6) as compared to GMD patients (n = 8) who subsequently during pregnancy needed treatment also with insulin. The insulin treated GDM group only had higher fasting glucose level than controls (5.2 vs 4.2 mmol/l, p < 0.001). Both GDM subgroups had slightly elevated basal levels of FFA and 3-hydroxybutyrate. Si and EIR were inversely correlated in control women and their fasting glucose correlated both to EIR (r = 0.63, p < 0.05) and to Si (r = 0.59, p < 0.05). In the GDM subgroups Si and EIR were unrelated and there were no correlations between fasting glucose and Si or EIR. These results suggest that glucose intolerance in GDM patients in the last trimester of pregnancy is characterized by both an impaired insulin secretion and an increased resistance to insulin. The impairment of insulin secretion and action increases with the severity of hyperglycemia, and the relative insulin deficiency characterizing GDM patients is associated with a selected defect in insulin action mainly affecting gluco-regulation.