The severity of illness in 58 surgical patients with high-grade intra-abdominal infection was measured with two methods, an acute physiology score and a septic severity score. Both methods are the summed weight of derangements in physiologic factors representing the function of the major organ systems of the body. Sixteen patients died (28%). Score values with both methods were significantly higher for nonsurvivors than for survivors. There was good interrelation between the methods, and the scores correlated better with mortality than did age, chronic disease, anatomy, or cause. Three risk levels were recognized, low, high, and intermediate, with respective mortality rates of less than 10%, greater than 80%, and approximately 45%. Three-fourths of the patients were assigned to the same risk group with both methods. The severity of illness in patients with intra-abdominal infection can suitably be measured with both methods.