Lack of uniform reporting of negative outcomes makes interpretation of surgical literature difficult. We attempt to define and classify negative outcomes by differentiating complications, sequelae, and failures. Complications and sequelae result from procedures, adding new problems to the underlying disease. However, complications are unexpected events not intrinsic to the procedure, whereas sequelae are inherent to the procedure. Failures are events in which the purpose of the procedure is not fulfilled. We propose a classification of complications based on four grades: Grade I complications are alterations from the ideal postoperative course, non-life-threatening, and with no lasting disability. Complications of this grade necessitate only bedside procedures and do not significantly extend hospital stay. Grade II complications are potentially life-threatening but without residual disability. Within grade II complications a subdivision is made according to the requirement for invasive procedures. Grade III complications are those with residual disability, including organ resection or persistence of life-threatening conditions. Finally, grade IV complications are deaths as a result of complications. To illustrate the relevance of the classification, we reviewed 650 cases of elective cholecystectomy. Risk factors for development of complications were determined, and the classification was also used to analyze the value of a modified APACHE II as a preoperative prognostic score. Both supported the relevance of the proposed classification. The advantages of such a classification are (1) increased uniformity in reporting results, (2) the ability to compare results of two distinct time periods in a single center, (3) the ability to compare results of surgery between different centers, (4) the ability to compare results of surgical versus nonsurgical measures, (5) the ability to perform adequate metaanalysis, (6) the ability to identify objective preoperative risk factors, and (7) the ability to establish preoperative prognostic scores.