Brain microbleeds, indicative of cerebral small-vessel disease, may occur with increased frequency in patients with hypertension. However, little is known about the relation of these abnormalities with blood pressure levels. We assessed the relation between ambulatory measured blood pressure and the presence of microbleeds in a cohort of hypertensive patients without a history of cerebrovascular disease. A total of 218 participants (110 males, age 52.5+/-12.6 years) underwent 24-hour ambulatory blood pressure monitoring twice (off-medication) and brain MRI to detect microbleeds and coexisting white matter hyperintensities. We performed logistic regression analyses to relate the following blood pressure components (based on both recordings) to microbleeds: the mean 24-hour, awake, and asleep blood pressures; nocturnal hypertension (asleep pressure >or=120/70 mm Hg); nocturnal blood pressure dipping. Models were adjusted for age and sex, and additionally for cardiovascular risk factors and white matter hyperintensities. We detected microbleeds in 35 participants (16.1%; 95% confidence interval, 11.1% to 21.0%). On average, each standard deviation increment in blood pressure, whether 24-hour, awake, or asleep, was significantly and independently associated with a 1.8- to 1.9-fold higher likelihood for microbleeds (all models P<0.05). Similarly, the adjusted odds ratio for microbleeds was 5- to 6-fold higher in subjects diagnosed with nocturnal hypertension (all models P<0.05). Microbleeds were not associated with nocturnal dipping. In conclusion, brain microbleeds are frequently found in hypertensive patients without a history of cerebrovascular disease, and are independently associated with higher daytime as well as night-time blood pressure levels.