Local septic complications in acute pancreatitis need to be exactly characterized and defined in order to develop improved concepts for their prevention, early diagnosis, and therapy. While up to now all local septic complications have been termed abscesses, the present study for the first time delineates the morphologic, clinical, and laboratory criteria needed to distinguish between two separate clinical entities: the infected necrosis (IN) and the pancreatic abscess (PA). IN is defined as a diffuse bacterial inflammation of necrotic pancreatic and peripancreatic tissue, but without any significant pus collections. On the other hand, the morphologic substrate of PA is a localized collection of pus surrounded by a more or less distinct capsula. IN becomes clinically evident during the early phase of acute pancreatitis (AP). The patients with IN present both the signs of sepsis and the laboratory findings of AP. Thus in these patients the most fulminant course of AP is observed; 51.8% and 35.7% of them have pulmonary or renal insufficiency, respectively. The mortality of the patients with IN is high and amounts to 32.1%. Pancreatic abscess, on the other hand, does not develop before the fifth week after onset of symptoms and after subsidence of the acute phase of pancreatitis. In these patients laboratory signs of AP-like amylasemia, hypocalcemia, hyperglycemia, and rise of LDH are rarely observed. Corresponding to the lack of pathophysiologic effects of AP per se, pulmonary and renal insufficiencies occur in only 33.3% and 16.7%, respectively, and mortality in these patients is 22.2%. While an abscess may readily be identified by computed tomography, the differentiation between IN and non-IN can be very difficult.(ABSTRACT TRUNCATED AT 250 WORDS)