This study used an adoption design to investigate the relationships among genetic background, environmental factors, and clinical outcome of attention deficit/hyperactivity, aggressivity, and adult antisocial personality (ASP) in a sample of 283 male adoptees. A biologic parent adjudged to be delinquent or to have an adult criminal conviction predicted increased attention deficit/hyperactivity in the adopted away sons, as well as increased adult ASP diagnosis. Aggressivity in the adoptee was predicted by attention deficit/hyperactivity, and aggressivity in turn predicted increased adult ASP. Environmental factors of socioeconomic status (SES), and psychiatric problems in adoptive family members correlated significantly with various clinical outcomes of aggressivity, attention deficit/hyperactivity, and ASP. The results suggest that attention deficit/hyperactivity should be considered a syndrome that has a variety of correlated behaviors, such as aggressivity, and that each of these correlated behaviors is influenced by different genetic and environmental factors and their interactions. Depending on the mix of factors, adult ASP can be one of the outcomes. © 1991.