Objective: The Preschool ADHD Treatment Study (PATS) was a NIMH-funded, six-center, randomized, controlled trial to determine the efficacy and safety of immediate-release methylphenidate (MPH-IR), given t.i.d. to children ages 3 to 5.5 years with attention-deficit/hyperactivity disorder (ADHD). Method: The 8-phase, 70-week PATS protocol included two double-blind, controlled phases, a crossover-titration trial followed by a placebo-controlled parallel trial. The crossover-titration phase's primary efficacy measure was a combined score from the Swanson, Kotkin, Atkins, M-Flynn, and Pelham (SKAMP) plus the Conners, Loney, and Milich (CLAM) rating scales; the parallel phase's primary outcome measure was excellent response, based on composite scores on the Swanson, Nolan, and Pelham (SNAP) rating scale. Results: Of 303 preschoolers enrolled, 165 were randomized into the titration trial. Compared with placebo, significant decreases in ADHD symptoms were found on MPH at 2.5 mg (p less than 0.01), 5 mg (p less than 0.001), and 7.5 mg (p less than 0.001) t.i.d. doses, but not for 1.25 mg (p less than 0.06). The mean optimal MPH total daily dose for the entire group was 14.2 plus or minus 8.1 mg/day (0.7 plus or minus 0.4 mg/kg/day). For the preschoolers (n = 114) later randomized into the parallel phase, only 21% on best-dose MPH and 13% on placebo achieved MTA-defined categorical criterion for remission set for school-age children with ADHD. Conclusions: MPH-IR, delivered in 2.5-, 5-, and 7.5-mg doses t.i.d., produced significant reductions on ADHD symptom scales in preschoolers compared to placebo, although effect sizes (0.4-0.8) were smaller than those cited for school-age children on the same medication. (Contains 1 figure and 2 tables.)