We performed this study to elucidate the role of nighttime versus daytime ambulatory blood pressure in predicting stroke and cardiac events. The International Collaborative Study of the Prognostic Utility of ABPM, which includes prospective cohort studies of ambulatory blood pressure monitoring (ABPM) from seven sites, was analyzed in this study. The incidence of stroke and cardiac events were evaluated for an average of 5.8 years. A cox proportional hazards model of adjusting for site, age, sex, BMI, total cholesterol, smoking, and history of antihypertensive medications was used for the analysis. Dipping was defined as the percentage decline in nighttime systolic blood pressure (SBP) relative to daytime SBP. Three hundred and eleven cardiac events and 318 strokes were seen during the follow up periods. Awake and sleep SBP were both significantly associated with both cardiac and stroke events. When the awake and sleep SBP were entered together in the model, awake SBP was more strongly associated with cardiac events than sleep SBP (chi2=12.4, d.f.=1, P=0.0004); conversely, sleep SBP (chi2=13.5, d.f.=1, P<0.0002) was more predictive for stroke events than awake SBP, although awake SBP also remained a significant predictor (chi2=7.03, d.f.=1, P=0.008). The amount of dipping was a significant inverse predictor of stroke [hazards ratio (HR) 0.81 per 10% increase in dipping, confidence interval (CI) 0.70-0.94, chi2=7.70, d.f.=1, P=0.006] but not of cardiac events. It should not be assumed that one summary measure of ambulatory blood pressure would be the best predictor of different clinical outcomes.