In this article, the authors investigated the effect of maternal smoking during pregnancy on behavioral problems, which were not mediated by lower birth weight, in offspring at 3 y of age. The authors used the Child Behavior Checklist for ages 2-3 y (CBCL/2-3; Achenbach, Edelbrock and Howell) to assess behavioral problems in 1,377 2- to 3-y-old healthy twin pairs. Soon after the birth of twins, the authors collected pre- and perinatal information, including smoking habits of the mother during pregnancy. The question "Did you smoke during pregnancy?" could be answered by choosing one of three possible options: (1) never, (2) sometimes, or (3) regularly. The authors analyzed the effect of maternal smoking on the Child Behavioral Checklist total score and on several subscale scores for first- and second-born twins separately, and they adjusted for the possible confounding effects of birth weight, socioeconomic status, maternal age, and having been breast- or bottle-fed. There was a significant effect of maternal smoking on so-called externalizing behavior problems (oppositional, aggressive, overactive), but not on internalizing behavior problems (withdrawn, depressed, anxious), in both first- and second-born twins. The authors primarily attributed the enhanced externalizing problems to increased aggression. Although boys had higher externalizing (and aggression) scores than girls, the effect of maternal smoking was the same for boys and girls. The authors also discuss whether maternal smoking causes externalizing behavior problems.