Objective: Long-term effects of psychostimulants on growth in height and in weight are investigated in children with attention-deficit/hyperactivity disorder. Method: Participants were 79 children, 6 to 12 years of age, with attention-deficit/hyperactivity disorder, who were followed annually for up to 5 years, between the years 1993 and 1994 and 1998 and 1999. Annual height and weight measurements were standardized by age and gender using the 2000 Centers for Disease Control and Prevention Growth Charts for the United States and reported as z scores. For children taking stimulants throughout the previous school year, dose potency was standardized to methylphenidate in milligrams per kilogram per day. We used hierarchical linear modeling to investigate the influence of dose and duration of stimulant treatment on the rate of growth in height and weight. Results: Controlling for time since initiation of treatment, daily dose of stimulant medication was negatively associated with z scores for height ([beta] = -0.11, SE = 0.03, p less than 0.01) and for weight ([beta] = -0.29, SE = 0.04, p less than 0.01). Estimates based on the statistical model suggest that children receiving greater than or equal to 1.5 mg/kg/day methylphenidate will show diminished weight gain after 1 year; those receiving greater than or equal to 2.5 mg/kg/day methylphenidate will show diminished gains in height after 4 years. Conclusion: Long-term use of high doses of stimulants during a period of 1 to 5 years is likely to have measurable effects on the rate of growth in school-age children with attention-deficit/hyperactivity disorder. (Contains 5 tables.)