Pancreatic necrosis is a principal determinant of the severity, duration, and infectious complications of acute pancreatitis. There has been no objective index for pancreatic necrosis, and its recognition has necessarily rested upon nonspecific clinical signs, including later deterioration or appearance of sepsis. In search of such an index, we have measured serum levels of a poly-[C]-specific acid ribonuclease (RNase) in 38 patients with acute pancreatitis, 12 patients with chronic pancreatitis, and 50 control patients. The values in chronic pancreatitis (mean, 52 units; range, 33 to 80 units) were within observed normal limits (mean, 51; range, 17 to 94). The values in acute pancreatitis segregated into two groups, normal values (group A) and high values (group B). Of 25 patients in group A (mean, 46; range, 19 to 87), only one developed evidence of pancreatic necrosis or abscess. In contrast, of the 13 patients in group B (mean, 192, range, 98 to 385), 11 required surgical debridement/drainage for pancreatic necrosis (six) or abscess (five) (P less than 0.001). Each of the other two patients had prolonged pancreatic inflammation with fever and a pancreatic mass which persisted for more than 2 weeks. RNase levels in group B patients rose within a few days after onset of pancreatitis and tended to parallel the clinical course. These findings suggest that measurement of serum RNase in acute pancreatitis gives a reliable indication of pancreatic necrosis. Therefore RNase determinations should be of value for earlier identification and monitoring of patients at high risk of late complications, and for helping to select those who will benefit from early debridement before secondary infection occurs.