We investigated the effects of maternal smoking during pregnancy on behavioral problems (i.e., not mediated by low birth weight) in 3-y-old offspring. We assessed behavioral problems in 1377 2- to 3-y-old twin pairs (registered in the Netherlands Twin Register) with the Child Behavior Checklist for ages 2-3 y (CBCL/2-3) from Achenbach, Edelbrock, and Howell. Two to 3 y earlier (i.e., soon after the birth of the twins) we collected information about the smoking habits (i.e., "never," "sometimes," and "regularly") of the mother during pregnancy. We analyzed the effect of maternal smoking on the CBCL total score and on several subscale scores for first- and second-born twins separately, and we adjusted for the possible confounding effects of birth weight, socioeconomic status, maternal age, and type of feeding (i.e., breast or bottle fed). There was a significant effect of maternal smoking on so-called "externalizing" behavioral problems (e.g., oppositional, aggressive, overactive), but not on "internalizing" behavioral problems (e.g., withdrawn, depressed, anxious), in both first- and second-born twins. The enhanced "externalizing" problems were attributed predominantly to increased aggression. Although boys have higher externalizing and aggression scores than girls, the effect of maternal smoking was the same for boys and girls.