The Japanese grading system for severity of acute pancreatitis has evolved from results of a national survey on 2,553 patients managed in 523 major medical centers in Japan between 1982 and 1986. It was devised to embrace the predictive factors that would offer high sensitivity and specificity in rendering an accurate diagnosis. In this system, computed tomographic (CT) examination is used for estimating the severity of acute pancreatitis. Combining criteria from laboratory data, clinical signs, and CT findings, the system appears cumbersome. However, it is actually more flexible in that only few criteria, from the wide range of findings actually covered, are needed to predict severity. Its rationale stems from a national effort to improve the outcome of this life-threatening disease. Further prospective studies are needed to determine its accuracy and superiority.