Recent clinical trials in hypertension suggest that thiazide diuretics may increase coronary heart deaths in selected patients, possibly through adverse effects on serum potassium, lipids, lipoproteins, and/or apolipoproteins. Administration of smaller doses of diuretics has been recommended to decrease this risk. We evaluated 12.5-mg and 112.5-mg daily doses of hydrochlorothiazide (HCTZ) administered for 1 month to nine postmenopausal black female hypertensives using a double-blind, randomized, crossover design. Both regimens produced significant reductions in sitting diastolic blood pressure, a mean of 11 mm Hg with the high dose and 8 mm Hg with the low dose. The high dose produced a mean 0.7 mEq/L reduction in serum potassium while the low dose caused no change. Both doses produced similar changes in serum lipoproteins. Statistically significant elevations were seen in total cholesterol (approximately 12%), LDL cholesterol (approximately 20%), cholesterol: HDL ratio (approximately 15%), and apolipoprotein B (approximately 20%). Apolipoprotein A1 was significantly reduced (approximately 6%). These results support the use of low doses of HCTZ in mild hypertension to avoid hypokalemia, but suggest that adverse changes in serum lipids will occur.