The hypolipidaemic effect of a low-fat, low-cholesterol diet, alone and in combination with weight reduction, has been evaluated in two groups of obese men with hypercholesterolaemia. In 41 men who lost 10.3 kg over 11 months and maintained their lower weight for 23.5 months serum cholesterol fell by 1.68 mmol/l and remained steady at lower weight. In 20 similar men, the controls, whose weight fell by 0.8 kg over 39.5 months, serum cholesterol fell by 0.80 mmol/l. There was a significant linear correlation between change in weight and change in serum cholesterol. The change in serum cholesterol in the weight losers was greater than could be accounted for by change in dietary lipids. Weight reduction and low-fat, low-cholesterol diets appear to have a synergistic effect in reducing serum cholesterol.