Acute pancreatitis is a protean disease capable of wide clinical variation, ranging from mild discomfort to apocalyptic prostration. Moreover, the inflammatory process may remain localized in the pancreas, spread to regional tissues, or even involve remote organ systems. This variability in presentation and clinical course has plagued the study and management of acute pancreatitis since its original clinical description. In the absence of accepted definitions for acute pancreatitis and its complications, it has not been possible to devise a clinical classification system useful for case management. Following 3 days of group meetings and open discussions, unanimous consensus on a series of definitions and a clinically based classification system for acute pancreatitis was achieved by a diverse group of 40 international authorities from six medical disciplines and 15 countries. The proposed classification system will be of value to practicing clinicians in the care of individual patients and to academicians seeking to compare interinstitutional data.