A 16-yr prospective study of 615 gestational diabetic subjects, half of whom were randomly assigned to insulin therapy during pregnancy, is evaluated to report on the potential reduction in subsequent diabetes due to insulin therapy. No such difference in incidence rates was observed between the two groups. In the subsets of women managed with insulin who bore a baby of large birthweight or who had a family history of diabetes, subsequent decompensated diabetes was found to be significantly reduced. Both life table and multivariate analyses to adjust for the effects of age, weight, level of blood glucose at selection, duration of follow-up, and other potentially confounding covariables confirmed this conclusion. The finding suggests the possibility of long-term preventive benefits from insulin treatment in high risk subsets of women with gestational diabetes.