The distribution of blood pressure (BP) values over the day and night was assessed in a group of 30 never previously treated patients with mild-to-moderate essential hypertension via 24-hour ambulatory BP monitoring. Elevated BP values during the awake hours (greater than 140/90 mm Hg) and sleeping hours (greater than 120/80 mm Hg) were used to calculate the total percentage of abnormal BP values (load) in each patient. The relationship between BP load and several indexes of hypertensive cardiac target organ involvement was compared to the relationships among office (casual), 24-hour average BP values, and cardiac indexes. Casual systolic and diastolic BP values did not correlate with left ventricular mass index, left atrial index, or peak left ventricular filling rate. Both 24-hour average BP and systolic and diastolic BP loads correlated with all indexes of cardiac target organ involvement. The BP loads were related to left ventricular mass index and left atrial index more strongly than were the mean 24-hour BP values; however, they were equally correlated for peak left ventricular filling rate. If greater than 40% of the ambulatory BP values were elevated, the likelihood of increased mass or decreased filling was greater than 61%, whereas if less than 40% of the BP values were elevated, the incidence of an abnormal cardiac test result decreased to less than 17%. These data show that the percentage of elevated BP values that includes both the awake and asleep periods is predictive of cardiac target organ involvement in patients with mild-to-moderate hypertension. Patients with mild hypertension who have more than 40% abnormal BP values should strongly be considered for antihypertensive therapy.