OBJECTIVE: To develop a tool for identifying and quantifying morbidity following cardiac surgery (cardiac postoperative morbidity score [C-POMS]). STUDY DESIGN AND SETTING: Morbidity was prospectively assessed in 450 cardiac surgery patients on postoperative days 1, 3, 5, 8, and 15 using POMS criteria (nine postoperative morbidity domains in general surgical patients) and cardiac-specific variables (from expert panel). Other morbidities were noted as free text and included if prevalence was more than 5%, missingness less than 5%, and mean expert-rated severity-importance index score more than 8. Construct validity was assessed by expert panel review, Cronbach's alpha (internal consistency), and linear regression (predictive ability of C-POMS for length of stay [LOS]). RESULTS: A 13-domain model was derived. Internal consistency (>0.7) on D3-D15 permits use as a summative score of total morbidity burden. Mean C-POMS scores were 3.4 (D3), 2.6 (D5), 3.4 (D8), and 3.8 (D15). Patient LOS was 4.6 days (P=0.012), 5.3 days (P=0.001), and 7.6 days (P=0.135) longer in patients with C-POMS-defined morbidity on D3, D5, D8, and D15, respectively, than in those without. For every unit increase in C-POMS summary score, subsequent LOS increased by 1.7 (D3), 2.2 (D5), 4.5 (D8), and 6.2 (D15) days (all P=0.000). CONCLUSION: C-POMS is the first validated tool for identifying total morbidity burden after cardiac surgery. However, further external validation is warranted.