OBJECTIVE: We evaluated the combined effect of hospital and surgeon volume on operative outcomes of mitral valve surgery in the United States. METHODS: The Nationwide Inpatient Sample was used to identify adult patients undergoing isolated mitral valve surgery for mitral regurgitation from 2003 to 2008. Hospitals and surgeons were separately stratified into equal-size tertiles according to annual overall mitral valve operative volumes. Multivariate logistic regression analysis was conducted, adjusting for multiple patient, hospital, and operative data, to determine the separate and combined effects of hospital and surgeon volume on operative outcomes. RESULTS: A total of 50,152 eligible patients were identified during the study period. Although both hospital and surgeon volume correlated significantly with operative mortality in separate risk-adjusted analyses, only lower surgeon volume persisted as a significant risk factor in the combined risk-adjusted analysis. Moreover, although hospital volume only accounted for 10.7% of the surgeon volume effect on increased mortality for low-volume surgeons, surgeon volume accounted for 74.5% of the hospital volume effect on increased mortality in low-volume hospitals. Surgeon, but not hospital, volume correlated with inpatient costs. Also, significant trends were seen with repair rates, with increasing surgeon volume demonstrating a relatively stronger correlation with the odds of repair (P < .001) than hospital volume (P = .01). CONCLUSIONS: The effect of hospital volume on operative outcomes of mitral valve surgery was largely driven by the individual surgeon volumes within that hospital. Conversely, surgeon volume affected these outcomes independently of hospital volume. Identifying the processes by which higher volume surgeons attain better outcomes in mitral valve surgery would therefore be prudent.