In a prospective clinical study including 114 patients with acute necrotizing pancreatitis, but excluding patients with a pancreatic abscess, necrotic material obtained at surgery was tested bacteriologically. Intestinal microorganisms were cultured in 39.4% of the cases. The contamination rate was 23.8% in patients operated on during the first 7 days of the attack; it rose to 71.4% in the third week and decreased to 32.5% after the fourth week. Intra- and extrapancreatic necrosis was more widespread and pancreatitis-associated ascites was more frequent in patients with proven contamination. The number of objective signs was 4.5 (median) and postoperative mortality was 37.8% in bacteriologically positive subjects, whereas the number was 3.5 (median) and mortality was 8.7% in bacteriologically negative patients. Morphologic and clinical alterations were more severe, and the mortality rate was significantly elevated, in patients with a short history of disease and bacterial contamination of necrotic tissue. All 5 patients with pancreatic sepsis who were operated on in the first 7 days of the disease, as compared with 2 of 16 patients with sterile necrosis, died. Thus, it is demonstrated that bacterial contamination of pancreatic necrosis occurs early and frequently, causing a significant increase in morbidity and mortality, particularly when it develops in the initial stages of the attack.