BACKGROUND: It is uncertain whether ambulatory blood-pressure measurements recorded for 24 hours in patients with treated hypertension predict cardiovascular events independently of blood-pressure measurements obtained in the physician's office and other cardiovascular risk factors. METHODS: We assessed the association between base-line ambulatory blood pressures in treated patients and subsequent cardiovascular events among 1963 patients with a median follow-up of 5 years (range, 1 to 66 months). RESULTS: We documented new cardiovascular events in 157 patients. In a Cox proportional-hazards model with adjustment for age, sex, smoking status, presence or absence of diabetes mellitus, serum cholesterol concentration, body-mass index, use or nonuse of lipidlowering drugs, and presence or absence of a history of cardiovascular events, as well as blood pressure measured at the physician's office, higher mean values for 24-hour ambulatory systolic and diastolic blood pressure were independent risk factors for new cardiovascular events. The adjusted relative risk of cardiovascular events associated with a 1-SD increment in blood pressure was 1.34 (95 percent confidence interval, 1.11 to 1.62) for 24-hour ambulatory systolic blood pressure, 1.30 (95 percent confidence interval, 1.08 to 1.58) for ambulatory systolic blood pressure during the daytime, and 1.27 (95 percent confidence interval, 1.07 to 1.57) for ambulatory systolic blood pressure during the nighttime. For ambulatory diastolic blood pressure, the corresponding relative risks of cardiovascular events associated with a 1-SD increment were 1.21 (95 percent confidence interval, 1.01 to 1.46), 1.24 (95 percent confidence interval, 1.03 to 1.49), and 1.18 (95 percent confidence interval, 0.98 to 1.40). CONCLUSIONS: In patients with treated hypertension, a higher ambulatory systolic or diastolic blood pressure predicts cardiovascular events even after adjustment for classic risk factors including office measurements of blood pressure.