Objective: This study examined associations between stimulant use and risk of cardiovascular events and symptoms in youth with attention-deficit/hyperactivity disorder and compared the risks associated with methylphenidate and amphetamines. Method: Claims were reviewed of privately insured young people 6 to 21 years old without known cardiovascular risk factors (n = 171,126). A day-level cohort analysis evaluated the risk of cardiovascular events after a diagnosis of attention-deficit/hyperactivity disorder in relation to stimulant exposures. Based on filled stimulant prescriptions, follow-up days were classified as current, past, and no stimulant use. Endpoints included an emergency department or inpatient diagnosis of angina pectoris, cardiac dysrhythmia, or transient cerebral ischemia (cardiac events) or tachycardia, palpitations, or syncope (cardiac symptoms). Results: There were 0.92 new cardiac events and 3.08 new cardiac symptoms per 1,000,000 days of current stimulant use. Compared with no stimulant use (reference group), the adjusted odds ratios of cardiac events were 0.69 (95% confidence interval 0.42-1.12) during current stimulant use and 1.18 (95% CI 0.83-1.66) during past stimulant use. The corresponding adjusted odds ratios for cardiac symptoms were 1.18 (95% CI 0.89-1.59) for current and 0.93 (95% CI 0.71-1.21) for past stimulant use. No significant differences were observed in risks of cardiovascular events (2.14, 95% CI 0.82-5.63) or symptoms (1.08, 95% CI 0.66-1.79) for current methylphenidate use compared with amphetamine use (reference group). Conclusions: Clinical diagnoses of cardiovascular events and symptoms were rare and not associated with stimulant use. The results help to allay concerns over the cardiovascular safety of stimulant treatment for attention-deficit/hyperactivity disorder in young people without known pre-existing risk factors. (Contains 4 tables.)