Although coronary heart disease is a major factor in morbidity and mortality in both sexes, most studies prior to 1988 when the study was initiated focused on men and overlooked the magnitude of the problem in women. Coronary heart disease mortality and morbidity rates are higher in men than in women, accounting for 41 percent of the sex difference in overall mortality in the United States. Women's advantage, however, does not seem to be stable over time nor universal. For example, in 1920, the age adjusted ratio of male to female coronary heart disease deaths was approximately equal, but between 1975 and 1978, it has increased to a level of 2.47 in the United States. Also, sex mortality ratios for heart disease differ widely across countries ranging from 5/1 in Finland to 1.87/1 in Yugoslavia. Thus, relying on biological differences alone when explaining this sex differential is not convincing.