A prospective cohort study compared the performance of administrative and clinical database risk adjustment models at a tertiary–quaternary care hospital. Observed mortality was 1.96%, which was the same as that predicted by the Society of Thoracic Surgeons model (1.96%), but significantly higher than that predicted by the Canadian Institute of Health Information (CIHI) model (1.03%). Despite both models having similar C statistics, the CIHI model showed significant underestimation of mortality among patients at higher risk. There was significant miscalibration of risk associated with 7 covariates. Together, these factors accounted for 84% of the difference in predicted mortality between the administrative and clinical models. Risk prediction using administrative data underestimated risk of death, potentially inflating observed-to-predicted mortality ratios at hospitals with patients who are more ill. Caution is warranted when hospital reports of cardiac surgery outcomes are based on administrative data alone.