The present study examined the power of measures of early preschool behavior to predict later diagnoses of attention-deficit/hyperactivity disorder (ADHD) and oppositional defiant disorder (ODD)/conduct disorder (CD). Participants were 168 children with behavior problems at age 3 who underwent a multimethod assessment of ADHD and ODD symptoms and were followed annually for 3 years. Fifty-eight percent of 3-year-old children with behavior problems met criteria for ADHD and/or ODD/CD 3 years later. Using a diagnostic interview and rating scales at age 3, the authors could accurately predict later diagnostic status for 3/4 of children for ADHD and for 2/3 of children for ODD/CD. Predictive power of the best models did not increase significantly at age 4 and age 5 compared with age 3. Results provide support for the validity of early diagnoses of ADHD, although caution is needed in making diagnoses because a significant minority of children with early hyperactivity and inattention do outgrow their problems.