Serum cholesterol and its subfractions were estimated in 23 patients on thiazide diuretics, for treatment of mild hypertension, for at least 30 months. The results were compared with those obtained in a matched set of 25 untreated mild hypertensives. It was found that while there was no difference in the level of total cholesterol, the LDL-cholesterol was significantly lower and HDL-cholesterol significantly higher in the treated group. It was also found that the total:LDL-cholesterol ratio was more in the treated group and the total:HDL-cholesterol ratio was less in the treated group. These findings suggest that coronary risk attributable to cholesterol and its subfractions does not seem to operate in long term thiazide treated patients.