The effect of weight reduction on circulating lipids was studied in 17 normotensive, normolipemic obese women consuming diets with differing carbohydrate contents. Ten subjects consumed an 800-calorie, 10-gram carbohydrate diet, and 7 subjects followed an 800-calorie, 70-gram carbohydrate diet. At the end of 12 weeks of dieting, the mean weight loss was 12.3 kg for both groups (P less than 0.001) but not significantly different between the two groups. Serum insulin decreased by 31% (P less than 0.05) and a reduction of 18% (P less than 0.025) was observed in serum triiodothyronine. Serum potassium and magnesium remained unchanged; however, serum magnesium was marginally low for the duration of the study. Only four nonspecific changes were observed in a total of 68 electrocardiograms (5.8%); there were no arrhythmias. The response of serum cholesterol was biphasic, decreasing initially and returning to the baseline by the twelfth week. The high-density lipid (HDL) cholesterol followed a pattern similar to that of total cholesterol. A possible mechanism explaining the changes in circulating lipids is proposed. The results indicate that, in spite of substantial changes in weight, the cholesterol-to-HDL cholesterol ratio remained unchanged. Although weight reduction did not alter the risk factor in a group of normotensive, normolipemic obese women, hypocaloric diets should be prescribed under medical supervision, especially when cardiovascular or other diseases coexist.