Despite the clinical importance of pancreatic necrosis in the course of acute pancreatitis, little is known about when it develops. Serum C-reactive protein (CRP) is a reliable parameter with a high deduction rate for pancreatic necrosis. We analyzed 199 patients with acute pancreatitis. The development of pancreatic necrosis was ascertained by a daily measurement of serum CRP in 45 patients with contrast-enhanced computed tomographic-proven necrotizing pancreatitis. In all 45 cases, the criteria for pancreatic necrosis were satisfied within the first 4 days of the onset of symptoms. This indicates that pancreatic necrosis is an early finding that develops within hours.