BACKGROUND: Morning blood pressure (BP) surge in ambulatory BP monitoring was a risk factor for stroke in our previous study. We studied the determinants of the morning minus evening systolic BP difference (ME difference) in self-measured BP monitoring, as a possible risk factor for stroke in medicated hypertensive patients. METHODS: Nine hundred sixty-nine hypertensive outpatients receiving stable antihypertensive drug treatment were studied using self-measured BP monitoring in the morning and evening. RESULTS: The ME difference ranged from -37.3 to 53.3 mm Hg (mean 7.9 mm Hg). The highest quartile (Q4) of the ME difference group (>15.0 mm Hg) had older age (68.0+/-9.8 years v 66.2+/-10.3 years, P=.01) and higher prevalence of men (48.3% v 39.9%, P=.02), regular alcohol drinkers (34.7% v 26.0%, P=.01) and beta-blocker use (26.9% v 19.9%, P=.03) than the other quartile groups (Q1 to Q3), whereas there was no significant difference in the average of morning and evening (ME average) BP. In logistic regression analysis controlling for ME average and other confounding factors, independent risks for Q4 of ME difference were older age (10 years older: odds ratio [OR] 1.21, P=.01, 95% confidence interval (CI) 1.04-1.42), regular alcohol drinker (OR 1.51, P=.04, 95% CI 1.01-2.26), and beta-blocker use (OR 1.50, P=.02, 95% CI 1.06-2.12). CONCLUSIONS: Older age, beta-blocker use, and regular alcohol drinking were significant determinants of the exaggerated ME difference in medicated hypertensive patients.