Microalbuminuria has been shown in approximately 40% of patients with essential hypertension. Previous studies have failed to demonstrate any consistent relationship between microalbuminuria and levels of office blood pressure. Because average ambulatory blood pressure correlates with incidence of cardiovascular morbidity and mortality better than office blood pressure, we have studied whether levels of urinary albumin excretion correlate with average diurnal, nocturnal, or 24-h blood pressure better than with office blood pressure. Sixty-three patients with essential hypertension and 21 healthy volunteers were included in the study. Twenty-four hypertensive patients failed to show the normal nighttime fall in blood pressure of at least 10/5 mm Hg and were defined as "nondippers"; the remaining were defined as "dippers." Office blood pressure was not different between dippers and nondippers. However, nighttime systolic and diastolic blood pressures were significantly greater in nondippers than in dippers. The median urinary albumin excretion in nondippers (42 mg/24 h) was significantly greater (P < .001) than in dippers (17.5 mg/24 h), and in normal subjects (8.6 mg/24 h). A significant correlation was present between nighttime systolic and diastolic blood pressure and urinary albumin excretion (UAE) and between 24-h systolic blood pressure and UAE in all hypertensive patients; in addition, a significant correlation was present between 24-h diastolic and nighttime diastolic blood pressure and UAE in nondippers. The increased amount of UAE in nondipper hypertensive patients suggests the presence of greater renal damage than in dippers.