The Collaborative Study in Coronary Artery Surgery (CASS) is a large multi-institutional study of the medical and surgical treatment of coronary artery disease. Fifteen cooperative institutions have carried out isolated coronary artery bypass grafting (CABG) on 6,258 men and 1,153 women during the period August, 1975, through May, 1980. The operative mortality in men was 1.9%, while the operative mortality for women undergoing CABG in the same institutions during the same time period was 4.5%. In an effort to explain this result, we used multivariate analysis to identify factors associated with increased mortality in women after CABG. The hypothesis that smaller physical size might be contributing to increased mortality was considered. Basic clinical and angiographic variables, size variables, including the average diameter of the grafted vessels, and gender were examined separately for patients who underwent elective and urgent and emergency procedures. Using multivariate information theory, we found that the most information regarding survival after operation is contained in basic clinical and angiographic variables. The physical size of the patient, including coronary artery diameter, helps predict operative mortality even after adjusting for differences in risk predicted by the basic variables and gender. However, the patient's sex is not statistically significantly related to the risk of surgical death given the information available from clinical and angiographic variables and from knowledge of patient size. One possible explanation of the excess risk for coronary artery operations in women is the smaller stature and the smaller diameter of the coronary arteries in this group of patients.