The antihypertensive effects of oral labetalol compared with placebo were evaluated in 74 mildly hypertensive patients (standing diastolic blood pressure 95 to 110 mm Hg) in a bicentric double-blind parallel group study. Following a four-week placebo phase, 36 patients were randomly assigned to receive labetalol and 38 to receive placebo. A five-week titration phase followed during which the dose of labetalol was increased weekly from 100 mg twice a day to 600 mg twice a day to achieve a standing diastolic blood pressure of less than 90 mm Hg and decreased 10 mm Hg or more from baseline. A matching number of placebo capsules for each dose of labetalol was dispensed for blinding purposes. Patients then entered a two-month maintenance phase. A thiazide diuretic could be added when the standing diastolic blood pressure was 100 mm Hg or greater at the highest dose of the study drug. At the end of this phase, the administration of labetalol (or placebo) was abruptly discontinued and patients were given the same number of placebo capsules twice a day taken during maintenance. Blood pressure and heart rate in the supine and standing position were measured eight to 10 hours after a dose at each visit. This study demonstrated that labetalol (median daily doses of 600 mg) was significantly more effective than placebo (p less than 0.05) in lowering the supine and standing blood pressures. Significantly more (p less than 0.001) placebo-treated patients than labetalol-treated patients (six versus 20) required the addition of a thiazide diuretic. Control of hypertension (that is, standing diastolic blood pressure less than 90 mm Hg) was achieved in significantly (p less than 0.001) more labetalol-treated patients than placebo-treated patients at the monotherapy endpoint (26 of 36; 72 percent versus six of 38; 16 percent). Blood pressure overshoot did not occur when labetalol was abruptly discontinued. Not one labetalol-treated patient discontinued the study because of adverse experiences. Labetalol is a safe and effective treatment for patients with mild hypertension.