OBJECTIVES: Socioeconomic status is associated with patients' health behaviors, health problems as well as the quality of care they receive. We examined data on U.S. elderly patients undergoing major orthopedic surgery to determine whether socioeconomic status was associated with cost of care for patients with similar access to health care provided by Medicare. METHODS: We used the 2004-2006 Medicare data to identify all patients undergoing major orthopedic surgery. We used previously constructed and validated SESScore ™, which is a summary measure of socioeconomic status for each U.S. zip code, using data from the 2000 U.S. Census. We assessed the effect of SESScore ™ on cost of care while controlling for other patient characteristics, and then examined the extent to which disparities in cost of care could be attributed to differences in hospital factors. Fixed effect models were used. RESULTS: SESScore ™ was a significant predictor of cost of care for major orthopedic surgery. Comparing the lowest quintile of SESScore ™ to the highest, the cost difference was positive and significant (p=0.000). After further adjustment for hospital factors, the difference was reduced. Within hospitals, there were only small differences in adjusted cost of care by patients' socioeconomic status. CONCLUSIONS: When undergoing major orthopedic surgery, patients with a lower socioeconomic status have higher rates of cost of care than patients with a higher socioeconomic status, despite similar insurance coverage provided by Medicare.