Objective: To examine the effects of high doses of extended-release methylphenidate (OROS MPH) on cardiovascular variables in adolescents with attention deficit hyperactivity disorder (ADHD). Study design: ECG indices plus systolic blood pressure (SBP), diastolic BP (DBP) and heart rate (HR) were assessed during an open-label study of OROS MPH in 114 adolescents with ADHD (doses up to 1.5 mg/kg/d). Cardiovascular parameters were assessed at 6 weeks and 6 months. Results: Small but statistically significant changes in DBP and HR were observed at 6 weeks, without further increases up to 6 months' follow-up. A small but statistically significant increase in SBP was observed over time. Twenty-nine percent of patients had isolated elevations in BP readings prior to study entry, and 14% had >3 consecutive visits at which elevated BP were observed during OROS MPH treatment. No clinically significant changes in ECG parameters were observed. No serious cardiovascular adverse events occurred. Conclusions: Treatment with relatively high doses of OROS MPH was associated with small but statistically significant mean increases in BP and HR, primarily during the first 6 weeks of treatment, without clinically meaningful changes in ECG. These observations are consistent with previous reports using lower doses.