In a prospective study of 367 patients referred for obstetric ultrasound evaluation, maternal cigarette smoking was found to have an important effect on the rate of placental maturation. The frequencies of grades 0, I, II, and III placentas throughout gestation were determined for both smoking and nonsmoking groups. Overall, the smoking mothers had more mature placentas than the nonsmoking mothers. This was reflected by the earlier mean gestational age at appearance of each of the placental grades in the smoking group as compared with the nonsmoking group. The mean gestational ages at appearance of grades 0, I, II, and III placentas for smoking mothers were 23.5, 27.9, 32, and 34.4 weeks, respectively, as compared with 26, 31.6, 35.7, and 38.3 weeks in the nonsmoking group. Smokers had a greater frequency of grade II placentas from 22-35 weeks than nonsmokers, and a greater frequency of grade III placentas beyond 25 weeks.