Glucose tolerance and insulin secretion were studied in 56 women 6-12 years following a pregnancy complicated by gestational diabetes, and in 23 matched controls. At recall 14 women were known to have diabetes and five were again pregnant with recurrent gestational diabetes. The early development of diabetes was associated with a fasting plasma glucose greater than 6 mmol/l during pregnancy and with a high plasma glucose response to oral glucose which persisted after delivery. Obesity was predictive of non-insulin-dependent diabetes whereas those that later required insulin were not obese. At recall, seven of the remaining 37 women were found to have unrecognized diabetes, 13 had impaired glucose tolerance (IGT) and 17 were normal by WHO criteria using a 75 g oral glucose tolerance test. In these 37 women, fasting plasma glucose and the glucose response to oral glucose in pregnancy were not predictive of subsequent diabetes or impaired glucose tolerance. Obesity in pregnancy and subsequent weight gain were associated with non-insulin-dependent diabetes and impaired glucose tolerance at recall. Insulin deficiency was observed during the oral glucose tolerance test in the diabetics (the mean +/- SEM ratio insulin area:glucose area 4.1 +/- 1.3 diabetics, 10.7 +/- 1.8 controls, p less than 0.05), whereas in the group with impaired glucose tolerance insulin levels were high and in proportion to their hyperglycaemia (insulin area:glucose area 10.9 +/- 1.4 IGT, 9.4 +/- 1.4 controls). Women with normal glucose tolerance and previous gestational diabetes had significantly lower insulin responses than their controls, despite mild hyperglycaemia (insulin area:glucose area 4.0 +/- 0.7 normal glucose tolerance, 7.6 +/- 1.1 controls, p less than 0.02). Abnormalities of glucose tolerance and insulin secretion are present following a gestational diabetic pregnancy. Gestational diabetes identifies women at risk for developing diabetes and impaired glucose tolerance, both of which are risk factors for premature vascular disease.