Data from a large prospective study of pregnancy were used to determine whether the number of years a mother had smoked cigarettes influenced the development of common fetal and placental disorders. Three disorders increased in frequency when mothers had smoked for more than 6 yr: placenta previa +143%, abruptio placentae +72% and large placental infarcts +37% (all P less than 0.05). Mothers' current smoking habits had a smaller influence on the frequency of these disorders, and the effects of smoking were largely independent of maternal pregnancy weight gain. The placentas of smokers had microscopic evidences of underperfusion from the uterus. The placental abnormalities were influenced by both the number of years mothers had smoked and by their current smoking habits.