Forty-two patients with mild-to-moderate hypertension between the ages of 35 and 65 (23 men, 19 women) were studied to determine whether psychological characteristics can help differentiate between responders and nonresponders to diuretic (hydrochlorothiazide 25 mg and triamterene 50 mg). To qualify for inclusion in the study, the subjects were required to have a mean unmedicated clinic diastolic blood pressure (DBP) between 95 and 110 mm Hg. Positive response to diuretic was defined as a reduction in clinic DBP < or = 90 mm Hg. Of the 42 subjects, 22 were responders to diuretic, achieving a BP level of 129/86 mm Hg, a reduction of 16/11 mm Hg from their unmedicated level. Nonresponders achieved a reduction of 8/4 mm Hg. Compared with nonresponders, responders were characterized by slightly lower initial BP levels and significantly lower scores on the Buss-Durkee Hostility Inventory and several subscales of this test. The pattern of results indicated higher levels of suppressed hostility in the nonresponders. Ambulatory BP data paralleled the clinic BP changes.