The Collaborative Multimodal Treatment Study of Children with Attention Deficit Hyperactivity Disorder (ADHD), the MTA, is the first child multisite cooperative agreement treatment study of children conducted by the National Institute of Mental Health, Rockville, Md. It examines the long-term effectiveness of medication vs behavioral treatment vs both for treatment of ADHD and compares state-of-the-art treatment with routine community care. In a parallel-groups design, 576 children (age, 7-9 years) with ADHD (96 at each site) are thoroughly assessed and randomized to 4 conditions: (1) medication alone, (2) psychosocial treatment alone, (3) the combination of both, (4) or community comparison. The first 3 groups are treated for 14 months and all are reassessed periodically for 24 months. Designers met the following challenges: framing clinically relevant primary questions; defining the target population; choice, intensity, and integration and combination of treatments for fair comparisons; combining scientific controls and standardization with clinical flexibility; and implementing a controlled clinical trial in a nonclinical setting (school) controlled by others. Innovative solutions included extensive decision algorithms and manualized adaptations of treatments to specific needs.