OBJECTIVE: This study investigated the role of maternal smoking during pregnancy in the etiology of attention deficit hyperactivity disorder (ADHD). METHOD: Subjects were 6-17-year-old boys with DSM-III-R ADHD (N = 140) and normal comparison subjects (N = 120) and their first-degree biological relatives. Information on maternal smoking was obtained from mothers in a standardized manner by raters who were blind to the proband's clinical status. RESULTS: Twenty-two percent of the ADHD children had a maternal history of smoking during pregnancy, compared with 8% of the normal subjects. This positive association remained significant after adjustment for socioeconomic status, parental IQ, and parental ADHD status. Significant differences in IQ were found between those children whose mothers smoked during pregnancy and those whose mothers did not smoke (mean IQ = 104.9, SD = 12.3, and mean = 115.4, SD = 12.2, respectively). CONCLUSIONS: These findings suggest that maternal smoking during pregnancy is a risk factor for ADHD. If confirmed, these findings will stress the importance of programs aimed at smoking prevention in nonsmoking women and smoking cessation in smoking women of childbearing age.