This is a longitudinal study of the relationship between prenatal tobacco exposure and the development of behavior problems in 672 children at the age of 3 years. Women from a prenatal clinic were interviewed about substance use at the end of each trimester of their pregnancy and at 3 years postpartum. Children were assessed at the age of 3 years with maternal ratings of behavior problems, activity, and attention. The prevalence of tobacco use was high in this cohort; 54.3% and 52.3% of the women smoked tobacco in the first and third trimesters of pregnancy, respectively. At 3 years postpartum, 61.6% of the women were smokers. There were significant effects of prenatal tobacco exposure on the children's behavior at age 3 years. Increases in scores on the Oppositional Behavior, Immaturity, Emotional Instability, Physical Aggression, and Activity scales and in the total score on the Toddler Behavior Checklist (TBC) were significantly associated with prenatal tobacco exposure. Smoking one pack of tobacco cigarettes per day during the third trimester of pregnancy was associated with an increase of 6 points in the total problem behavior score. Among the subscales of the TBC, tobacco exposure had the largest effect on oppositional behavior. Impulsivity and peer problems were associated with both prenatal and current tobacco exposure. Only current tobacco exposure predicted attention problems. Prenatal tobacco exposure had a significant negative effect on the development of behavior problems among preschoolers.