Epidemiological observations have suggested a relationship between type 2 diabetes and a low or high birth weight. However, there are many confounding variables and problems with retrospective data collection. The study of women with gestational diabetes mellitus (GDM), who are likely to develop type 2 diabetes in the future, may help clarify these observations. Women diagnosed as suffering from GDM (n=162) were included in the study if their own birth weight data were available. The birth-weight distribution of the general population was obtained from published data for the same period. A family history of diabetes prevalence was assessed from medical records for the 162 GDM subjects, and by direct interview of 250 non-diabetic pregnant controls. The birth-weight distribution of women with GDM, when normalized to that of the general population, showed a statistically significant U-shape with a greater proportion of GDM women having low (1000-2000 g) or high (>4500 g) birth weight. Compared to non-diabetic pregnant controls, a statistically higher proportion of GDM women had a maternal family history of diabetes; the mean birth weight of these women was also statistically higher than that of GDM women with no family history of diabetes. No such differences were noted among women with a paternal family history. These data suggest that the intrauterine millieur Interieur, whether one of nutritional deprivation or one of nutritional plenty, results in changes in pancreatic development and peripheral response to insulin that may lead to adult-onset GDM and type 2 diabetes. Genetic predisposition, unless determined by mitrochondrial genetic material, does not apparently have any part in determining birth weight.