Objective: To describe the rationale and design of the Preschool ADHD Treatment Study (PATS). Method: PATS was a National Institutes of Mental Health-funded, multicenter, randomized, efficacy trial designed to evaluate the short-term (5 weeks) efficacy and long-term (40 weeks) safety of methylphenidate (MPH) in preschoolers with attention-deficit/hyperactivity disorder (ADHD). Three hundred three subjects ages 3 to 5.5 years old who met criteria for a primary DSM-IV diagnosis of ADHD entered the trial. Subjects participated in an 8-phase, 70-week trial that included screening, parent training, baseline, open-label safety lead-in, double-blind crossover titration, double-blind parallel efficacy, open-label maintenance, and double-blind discontinuation. Medication response was assessed during the crossover titration phase using a combination of parent and teacher ratings. Special ethical considerations throughout the trial warranted a number of design changes. Results: This report describes the design of this trial, the rationale for reevaluation and modification of the design, and the methods used to conduct the trial. Conclusions: The PATS adds to a limited literature and improves our understanding of the safety and efficacy of MPH in the treatment of preschoolers with ADHD, but changes in the design and problems in implementation of this study impose some specific limitations that need to be addressed in future studies. (Contains 2 tables.)