Hypothesis: The 48-hour APACHE (Acute Physiology and Chronic Health Evaluation) II score is a better predictor of pancreatic necrosis, organ failure, and mortality in patients with severe acute pancreatitis than the score at hospital admission. Design: A retrospective analysis of 125 patients with acute pancreatitis. Setting: A tertiary public teaching hospital. Patients: Patients with severe acute pancreatitis as defined by 3 or more Ranson criteria or a hospital stay of longer than 6 days. Main Outcome Measures: Pancreatic necrosis, organ failure, and mortality. Results: A significant association was found between the 48-hour score and the presence of pancreatic necrosis (P<.001), organ failure (P=.001), and death (P<.001). By contrast, the APACHE II score at admission was significantly associated only with the presence of organ failure (P=.007). Deteriorating APACHE II scores over 48 hours were significantly associated with a fatal outcome (P=.03). The combined APACHE II score (defined as the sum of the admission and 48-hour scores) was significantly higher among nonsurvivors than survivors (P<.001), and was strongly associated with the presence of pancreatic necrosis (P=.001) and organ failure (P<.001). The 48-hour and combined scores accurately predicted outcome in 93% of the patients compared with 75% by the admission score. Conclusions: The 48-hour APACHE II score has improved predictive value compared with the admission score for identifying patients with severe acute pancreatitis who have a poor outcome. A deteriorating APACHE II score at 48 hours after admission may identify patients at risk for an adverse outcome.