The main problem in the diagnosis of acute pancreatitis is the early detection of the fulminant forms. So far, there is no single laboratory test that affords an unequivocal measure of the severity of the disease. We have studied the serum phospholipase A2 concentrations of 66 consecutive patients with acute or chronic relapsing pancreatitis. Fifty-two patients had a mild spontaneously healing form of the disease, whereas eight patients developed a pseudocyst or an abscess. Six patients had operatively established haemorrhagic pancreatitis. The serum phospholipase A2 concentrations were significantly higher in the patients with haemorrhagic pancreatitis than in those with a spontaneously healing disease. Moreover, in the patients with a milder form of the disease the serum phospholipase A2 concentrations correlated with prognostic signs suggested by others. The results suggest that the assaying of serum phospholipase A2 might be a valuable tool in the diagnosis and follow-up study of patients with acute pancreatitis.