Reactive C Protein (RCP) has been determined in fifty-one patients with acute pancreatitis. RCP has been compared with Ranson's criteria which include several clinical and biochemical parameters. The value of 20 mg/dl is the one which discriminates serious acute pancreatitis from the mild forms in a 84.3% of patients. Sensitivity and specificity of RCP and Ranson's criteria are compared, the results obtained are similar (88.9% vs. 81.8% and 94.4% vs. 97%). In summary, the determination of RCP is very useful for its simplicity and accuracy in the prognosis of acute pancreatitis.