Objective: To determine if in-hospital mortality after cardiac surgery can be predicted, in children, using a new clinical and surgical index. Study design: Observational, retrospective, cross-sectional. Methods: We reviewed 818 charts from children undergoing surgery between January, 2000, and December, 2004. The index was calculated by summing the scores from five variables, specifically age, nutritional state, the presence of associated clinical risk factors, surgical complexity, and use and time of cardiopulmonary bypass. Each variable was subdivided into categories of low, medium or high risk, with scores attributed as zero, one or two, respectively. Risks for death were calculated using the odds ratio. Results: Our overall mortality was 14.7%, with our proposed index correlating strongly with mortality (p less than 0.0001). No patients died with scores of zero, but mortality increased from around 10% with a score of three, to close to 30% with scores of five and six, and to over 50% with a score of eight. No patients reached scores of 10, and more than three-fifths of all patients had scores between zero and three. We observed higher mortalities independently for each variable in association with the highest risk scores. Conclusions: We found that surgery undertaken in the neonatal period, weight below the 5th percentile, the presence of associated clinical risk factors, operations of higher complexity, and more than 90 minutes of cardiopulmonary bypass were all significantly associated with mortality. Our suggested new index showed a linear correlation with mortality, and in our current experience, has proved a valuable tool for predicting adverse outcomes.