To examine differences in home, school, and medical functioning between preschool-age children with attentiondeficit/hyperactivity disorder (ADHD) and normal control children. A sample of 94 children (58 with ADHD, 36 normal controls) between 3 and 5 years old participated. Dependent measures included parent and teacher ratings of problem behavior and social skills, parent ratings of stress and family functioning, medical functioning data, observations of parent-child interactions and classroom behavior, and a test of preacademic skills. Young children with ADHD exhibited more problem behavior and were less socially skilled than their normal counterparts according to behavior ratings. Parents of children with ADHD experienced greater stress and were coping less adaptively than parents of non-ADHD children. Children with ADHD exhibited more noncompliant and inappropriate behavior than normal controls, particularly during task situations. Parents of children with ADHD were more likely to display negative behavior toward their children. Children with ADHD exhibited more negative social behavior in preschool settings and scored significantly lower on a test of preacademic skills. No significant differences in injuries or utilization of medical services were found. Preschool-age children with ADHD are at significant risk for behavioral, social, familial, and academic difficulties relative to their normal counterparts.