BACKGROUND Coronary artery bypass graft surgery (CABG) in women has been associated with worse clinical outcomes than CABG in men. However, little is known about the impact of sex on the cost of CABG. OBJECTIVE To examine the impact of sex on hospital course and the cost of CABG. METHODS Hospital course and cost were examined among 2880 female and 9137 male patients from four Canadian and five American hospitals. Data were obtained from a resource and cost accounting system used by each of the nine hospitals. RESULTS Among the 12,017 patients who underwent CABG, 24% (n=2880) were women and 76% (n=9137) were men. Women had a significantly longer length of stay (LOS) than did men (10.3+/-0.2 days and 8.9+/-0.08 days, respectively; P<0.0001) and a significantly higher in-hospital mortality than did men (2.6% and 1.5%, respectively; P<0.0001). The total unadjusted cost was higher for women than for men both in Canada (US$11,200+/-268 and US$10,143+/-139, respectively; P<0.0001) and the United States (US$22,715+/-509 and US$19,906+/-269, respectively; P<0.0001). After adjusting for age and comorbid conditions, female sex was associated with a 10% increase in LOS (P<0.0001), a 97% increase in mortality (P=0.0006) and a 7% increase in overall cost (P<0.0001). CONCLUSIONS Compared with men, women undergoing CABG had a modestly increased LOS and a higher mortality. Total in-hospital cost was higher for women in each of the nine hospitals studied. Compared with other clinical variables, female sex is a relatively minor determinant of cost. Nevertheless, because of the expected increase in the number of women undergoing CABG in the future, this increased cost may translate into an important economic burden.