We studied the relation in Pima Indians between obesity in children and diabetes during pregnancy in their mothers. Sixty-eight children of 49 women who had had diabetes during pregnancy had a higher prevalence of obesity than 541 children of 134 women who subsequently had diabetes (prediabetics) or than 1326 children of 446 women who remained nondiabetic. At 15 to 19 years of age, 58 per cent of the offspring of diabetics weighed 140 per cent or more of their desirable weight, as compared with 17 per cent of the offspring of nondiabetics and 25 per cent of those of prediabetics (P less than 0.001). Obesity in the offspring was directly related to maternal diabetes, since the association was not substantially confounded by maternal obesity. The findings strongly suggest that the prenatal environment of the offspring of diabetic women results in the development of obesity in childhood and early adulthood.