Objective: To report on the safety and tolerability of methylphenidate (MPH) 3- to 5-year-old children with attention-deficit/hyperactivity disorder (ADHD) during 1 year of treatment. Method: Exactly 183 children (3-5 years old) entered a treatment study of MPH, consisting of a 1-week open-label lead-in (n = 183); a 5-week placebo-controlled, double-blind phase (n = 165); a 5-week double-blind, parallel phase (n = 114); and 10 months of open-label maintenance (n = 140 entered, 95 completed). Mean total daily MPH doses rose from the titration trial best dose, 14.1 (plus or minus 8.1) mg/day, to 20.5 (plus or minus 9.7) mg/day mean total daily dose at the end of maintenance. Pulse, blood pressure, and the presence of treatment emergent adverse events (AEs), parent and teacher AE ratings, and vital signs were recorded in each phase. Results: Thirty percent of parents spontaneously reported moderate to severe AEs in all study phases after baseline. These included emotional outbursts, difficulty falling asleep, repetitive behaviors/thoughts, appetite decrease, and irritability. During titration, decreased appetite (chi square = 5.4, p less than 0.03), trouble sleeping (chi square = 5.4, p less than 0.03), and weight loss (chi square = 4.0, p less than 0.05) occurred statistically more often on MPH than on placebo. During maintenance, trouble sleeping and appetite loss persisted and other MPH-related AEs decreased. There were transient, one-time pulse and blood pressure elevations in five children. Twenty-one children (11%) discontinued because of drug-attributed AEs. Conclusions: Eleven percent of preschoolers discontinued treatment because of intolerable MPH AEs. Of the serious AEs reported, one occurred in baseline, two in lead-in, three in titration, one in parallel, and one in maintenance. Only one was possibly related to MPH. (Contains 1 figure and 2 tables.)