The authors considered whether the difference in body fat distribution between men and women, measured as waist:hip ratio, might explain part of the sex difference in coronary heart disease incidence in prospective population studies of 1,462 women and 792 men. In these studies, conducted in Sweden, men were found to have about four times higher odds for coronary heart disease than women during a 12-year follow-up period (men, 1967 to 1979; women, 1968-1969 to 1980-1981). Controlling for differences in blood pressure, serum cholesterol, smoking, and body mass index only marginally altered the magnitude of the male-female difference. When waist:hip ratio, which predicted coronary heart disease rates in both sexes, was also considered, the sex difference in coronary heart disease risk was significantly reduced and virtually disappeared (odds ratios = 1.0-1.1; nonsignificant). The findings suggest that body fat distribution or a factor highly correlated with waist:hip ratio (genetic, hormonal, or behavioral) may help to explain the sex differences in coronary heart disease.