1. The serum concentrations of cholesterol and triglycerides were measured before and during the treatment of hypertension. Thirty-six patients were treated with hydrochlorothiazide and 29 with spironolactone.
2. Serum cholesterol increased from 214 to 227 mg/100 ml (P < 0·005) and triglyceride did not change during treatment with hydrochlorothiazide. Serum triglyceride increased from 112 to 133 mg/100 ml (P < 0·05) and cholesterol did not change during treatment with spironolactone.
3. Serum lipid responses to diuretics varied among patients. To identify patients susceptible to larger increases, subgroup analysis was carried out. Patients who were younger and had lower pretreatment serum cholesterol and systolic blood pressure had larger increments in cholesterol during treatment. In these subgroups coronary risk status did not improve during therapy.
4. The incidence of myocardial infarction may not be reduced so long as diuretics serve as first-line drugs in the treatment of mild hypertension.