We assessed the predictive validity of attention-deficit/hyperactivity disorder (ADHD) in 20 girls and 98 boys who met the Diagnostic and Statistical Manual for Mental Disorders (4th ed., American Psychiatric Association, 1994) criteria for ADHD at 4 to 6 years of age compared to 24 female and 102 male comparison children. Over the next 8 years, both girls and boys who met criteria for ADHD in Year 1 exhibited more ADHD symptoms and impairment than same-sex comparison children. Effect sizes were consistently large, indicating that the diagnosis of ADHD at 4 to 6 years of age has predictive validity for both sexes. Both girls and boys with ADHD in Year 1 also exhibited higher levels of symptoms of conduct disorder, major depression, and anxiety disorders in early adolescence than same-sex comparison children, controlling levels of the same symptoms in Year 1. This indicates both substantial homotypic and heterotypic continuity for ADHD in both sexes, but significant interactions with time indicated that childhood ADHD predicts more steeply rising symptoms of anxiety and depression during early adolescence in girls than in boys.