Medical records of 1,207 Pima Indian children were examined for reported congenital anomalies. Anomalies occurred in eight (38.1 per cent) of twenty-one offspring born after the onset of diabetes to mothers whose disease was diagnosed before age twenty-five, but in only 3.7 per cent of the offspring of all other women. Children born after the onset of diabetes to mothers whose disease started at or after age twenty-five, and those born to prediabetic mothers had anomalies no more frequently than the children of nondiabetic mothers. Congenital anomalies were not related to paternal diabetes. Anomalies were more frequent in children from “diabetic” pregnancies during which the mother required hypoglycemic medication than from those during which medication was not required. Although a genetic mechanism cannot be completely excluded, the data better support the hypothesis that diabetes produced fetal anomalies among the Pima Indians by its influence upon the intrauterine environment during early pregnancy.