The antihypertensive efficacy and safety of doxazosin, a selective alpha 1-inhibitor, were assessed in 33 severely hypertensive patients. The study involved three phases: a 1-day baseline period, a 32-day period in which patients received doxazosin, 1 to 16 mg, once daily, and a 4-week maintenance period. All patients were considered therapy successes (sitting diastolic blood pressure either less than or equal to 90 mm Hg or greater than or equal to 10 mm Hg reduction) at a mean daily dose of 8.1 mg. Twenty-seven patients (82%) achieved good blood pressure control (sitting diastolic blood pressure less than or equal to 90 mm Hg) at a mean dose of 6.7 mg once daily. By the final treatment visit, mean systolic/diastolic blood pressures in the sitting and supine positions were significantly reduced by 40/29 and 37/28 mm Hg from a mean baseline of 180/121 and 181/121 mm Hg to final visit values of 140/91 and 144/92 mm Hg, respectively (p less than 0.05). Eleven patients experienced one or more side effects that were mild or moderate and disappeared or were tolerated with continued therapy. During the study no abnormal laboratory test findings were identified by the investigators. The investigators' global assessment of the efficacy and toleration of once-daily doxazosin therapy was excellent or good for 31 (94%) patients and fair for the remaining two patients.