The hypothesis that lean persons are more responsive than fat persons to the effects of dietary cholesterol was investigated in 1,903 middle-aged employed men who were examined in 1958 and 1959 as participants in the Chicago Western Electric Study. Change in intake of dietary cholesterol was positively associated with change in serum cholesterol for men in the lowest tertile of body mass index (defined as weight (kg)/height (m)2) (< 24.2) but not for men in the highest tertile (> 26.6) after adjustment for change in body mass index and change in intakes of energy and saturated and polyunsaturated fatty acids. A decrease of 150 mg/1,000 kcal in dietary cholesterol was associated with mean changes of -0.46, -0.18, and 0.13 mmol/liter in serum cholesterol for men with body mass indices of < 24.2, 24.2-26.6, and > 26.6, respectively. Body mass index was strongly correlated with subscapular skinfold thickness; thus, these differences in body mass index reflect true differences in adiposity. These results may help to explain inconsistencies that have occurred in feeding experiments with dietary cholesterol, and they suggest that a reduction in dietary cholesterol should have a more favorable effect on the serum cholesterol levels of fat persons after they have lost weight.