The APACHE II severity of disease classification system has been examined prospectively in 160 patients with acute pancreatitis. Using clinical and simple laboratory data APACHE II was able to provide useful discrimination between uncomplicated, complicated and fatal attacks within a few hours of admission. Peak APACHE II scores (recorded during the first 3 days) had a prognostic accuracy similar to the multiple factor scoring systems, but then incurred a similar delay. Patients could be graded according to their risk of death or of developing a major complication; no deaths occurred in patients with a peak APACHE II score less than 10. APACHE II can be repeated daily, uncomplicated attacks demonstrating falling scores in association with clinical improvement, in contrast to the rising scores associated with clinical deterioration in those dying early. APACHE II appears to reflect any continuing disease activity and may prove a useful means of monitoring the course of the illness and response to therapy.