Objective: This study examined the effectiveness of stimulant medication on multiple domains of functioning in 36 young (5 to 6 years old) children diagnosed with attention-deficit/hyperactivity disorder (ADHD). Method: Five- and 6-year-old children attending a summer treatment program between 1987 and 1997 underwent a randomized clinical assessment of the effect of two doses of methylphenidate (0.3 mg/kg versus 0.6 mg/kg) and placebo on social behavior and academic performance. Results: Methylphenidate had an effect on all four social behaviors and improved two of the three areas of academic functioning. Dose effects were present for three of the seven dependent measures. Individual analyses indicated a therapeutic response rate between 39% and 100% across dependent measures. Furthermore, individual analyses of response indicated that across several important dependent measures, 39% to 98% of children showed little incremental improvement with the higher dose compared with the lower dose of stimulant medication. Conclusions: Stimulant medication is an effective treatment for young children diagnosed with ADHD; however, multiple domains of functioning must be assessed to determine the most effective dose for young children with ADHD.