It remains unknown whether diabetes and high blood pressure (BP) are simply additive risk factors for cardiovascular outcome or whether they act synergistically and potentiate one another. We performed 24-h ambulatory BP monitoring in 8494 subjects (mean age, 54.6 years; 47.0% women; 6.9% diabetic patients) enrolled in prospective population studies in 10 countries. In multivariable-adjusted Cox regression, we assessed the additive as opposed to the synergistic effects of BP and diabetes in relation to a composite cardiovascular endpoint by testing the significance of appropriate interaction terms. During 10.6 years (median follow-up), 1066 participants had a cardiovascular complication. Diabetes mellitus as well as the 24-h ambulatory BP were independent and powerful predictors of the composite cardiovascular endpoint. However, there was no synergistic interaction between diabetes and 24-h, daytime, or nighttime, systolic or diastolic ambulatory BP (P for interaction, 0.07¿P¿0.97). The only exception was a borderline synergistic effect between diabetes and daytime diastolic BP in relation to the composite cardiovascular endpoint (P=0.04). In diabetic patients, with normotension as the reference group, the adjusted hazard ratios for the cardiovascular endpoint were 1.35 (95% confidence interval (CI), 0.87-2.11) for white-coat hypertension, 1.78 (95% CI, 1.22-2.60) for masked hypertension and 2.44 (95% CI, 1.92-3.11) for sustained hypertension. The hazard ratios for non-diabetic subjects were not different from those of diabetic patients (P-values for interaction, 0.09¿P¿0.72). In conclusion, in a large international population-based database, both diabetes mellitus and BP contributed equally to the risk of cardiovascular complications without evidence for a synergistic effect.Hypertension Research advance online publication, 10 February 2011; doi:10.1038/hr.2011.6.