Objective: To examine the long-term effects of the methylphenidate transdermal system (MTS) on the growth of children being treated for attention-deficit/hyperactivity disorder. Method: Height, weight, and body mass index (BMI) were measured in 127 children ages 6 to 12 at longitudinal assessments for up to 36 months of treatment with MTS. These data were compared with norms provided by the Centers for Disease Control and Prevention. Results: MTS treatment was associated with small but significant delays in growth for height, weight, and BMI. The latter two indices were affected in a dose-dependent manner. Children who had not received prior stimulant therapy and children who entered the study with above-average height, weight, and BMI were most likely to experience growth deficits during the trial. Effects on all parameters of growth were most apparent during the first year of treatment, and attenuated over time. Conclusions: Consistent with prior studies of methylphenidate, our results suggest that treatment with MTS can lead to reductions in expected height, weight, and BMI that show some attenuation over the course of treatment. Growth of patients with attention-deficit/hyperactivity disorder treated with MTS should be closely monitored, but in this study, deficits in growth in relation to MTS treatment were not a significant clinical concern for most children. (Contains 5 figures.) [This research was supported by Shire Pharmaceutical Development.]