Investigated the role of maternal smoking during pregnancy in the etiology of attention deficit hyperactivity disorder (ADHD). Siblings of ADHD (N = 174) and non-ADHD (N = 129) probands were studied. Information on maternal smoking was obtained from mothers in a standardized manner blind to the sibling's ADHD and high-risk status (i.e., whether a sibling of an ADHD or non-ADHD proband). Fifteen (47%) of the high-risk siblings with ADHD had a history of maternal smoking during pregnancy compared with 33 (24%) of the siblings without ADHD (p = 0.009). This positive association remained significant after adjusting for socioeconomic status, parental IQ, and parental ADHD status. Lower IQ scores were found among those high-risk siblings whose mothers smoked during pregnancy compared with those whose mothers did not smoke. These findings extend our previous findings of an association between maternal smoking during pregnancy and ADHD. Moreover, they highlight the importance of programs aimed at smoking prevention in nonsmoking women and smoking cessation in smoking women of child-bearing age.