A light microscopic, an electron microscopic, and a ultrahistochemical study have been made of seven placentas from women with mild and excellently controlled chemical gestational diabetes. Only one placenta showed no significant abnormality; the remainder showed, to a varying degree, changes in the syncytiotrophoblast, cytotrophoblast, trophoblastic basement membrane, and fetal vessels. These abnormalities were identical to and in some cases as severe as those found in placentas from women with longstanding, moderately controlled, overt diabetes mellitus. These findings indicate that control of hyperglycemia only partially prevents the development of placental abnormalities which must be due to some other constituent factor of the diabetic state.