Non-invasive ambulatory blood pressure monitoring was performed in a consecutive series of 87 subjects with recently detected mild uncomplicated hypertension. Obese subjects, diabetics and those with secondary hypertension were excluded. Ambulatory pressures were recorded on a day of usual activity. Average ambulatory systolic and diastolic pressures were significantly lower than referral pressures determined in clinics or screening sites and initial pressures taken by the monitors. Whereas men (57) and women (30) had similar referral and initial pressures, average ambulatory systolic pressure was significantly higher in men; diastolic pressure was not different. Men also had a significantly higher fraction of ambulatory systolic pressures greater than 140 mmHg compared to women. Fifty-six percent of the men and 80% of the women had average ambulatory systolic pressures less than 140 mmHg and diastolic pressures less than 90 mmHg; the difference between the sexes was significant (chi 2 = 6.99, P less than 0.01). Thus, in mild hypertension, women have lower average systolic pressure than men during ordinary daily activity. These results may account for lower long-term cardiovascular morbidity in hypertensive women compared with men.