Correlations were made between weight change (as an index of volume loss) and blood pressure (BP) reduction before and after hydrochlorothiazide treatment. A total of 343 patients with mild to moderate hypertension (95-114 mm Hg) received hydrochlorothiazide alone. The diuretic was titrated from 50 to 100 to 200 mg daily as needed until the diastolic BP fell below 90 mm Hg (goal BP) or side effects supervened. Of the 305 patients who completed the 10-week titration period, 65% attained goal BP. The effective dose of hydrochlorothiazide in 52% of these responders was 50 mg/day, and this was associated with weight loss averaging 1.58 kg. An additional 29% achieved goal BP with a similar degree of weight loss, but they required double the dose, or 100 mg/day. The remaining 19% of responders required significantly greater weight reductions averaging 3.14 kg to achieve goal BP, which necessitated hydrochlorothiazide, 200 mg/day. More blacks than whites attained goal BP despite similar degrees of weight loss in the two races. Plasma renin activity was initially higher in whites than in blacks and rose significantly more in blacks and whites requiring the greatest volume losses and the highest dose of hydrochlorothiazide to attain goal BP. Nonresponders had less weight loss than responders. Thus, diuretic dose requirements vary in different patients and are related either to different volume losses in response to a given dose or to different degrees of BP reduction in response to the same volume loss.