In a study attempting to improve coronary risk status, serum cholesterol and triglyceride levels were measured before and during treatment of 74 patients with mild primary hypertension. In 35 patients there was a satisfactory reduction in elevated blood pressure levels with diet therapy alone. In the remaining 39 patients a diuretic drug was required in addition to the diet. Diet therapy alone was followed by a decrease of 11 mg/100 ml in mean serum cholesterol (p less than 0.01 versus pretreatment value) and no change in serum triglyceride. The sue of diuretics was accompanied by an average increase of 11 mg/100 ml in serum cholesterol and of 34 mg/100 ml in serum triglyceride (p less than 0.01 versus pretreatment level for both). In a subgroup of 21 patients with greatest elevations in lipid levels during the administration of diuretics, little improvement in coronary risk status occurred because the increase in serum cholesterol balanced the decrease in systolic blood pressure, according to Framingham risk tables. If the level of serum lipids is a factor in the pathogenesis of coronary atherosclerosis then the observed effect of diuretic drugs to elevate serum cholesterol and triglyceride levels may explain, in part, the continuing high rate of occurrence of myocardial infarction during the treatment of hypertension.