The usefulness and clinical applicability of quantitative plasma polymorphonuclear elastase determinations in the diagnosis of the severity of acute pancreatitis was analysed in a multicentre study and was compared with the usual prognostic systems of Ranson and Osborne et al. The study comprised 182 patients, 154 with a mild episode of acute pancreatitis and 28 with a severe episode, defined by the development of major complications or a fatal outcome. In the severe cases neutrophilic elastase reached significantly higher values than in mild cases (P less than 0.001) by the time the patient was admitted (2-12 h after the onset of the disease), reflecting considerable leucocyte activation. The sensitivity and specificity of this test are therefore greater than 90 per cent, with a positive severity predictive value of almost 80 per cent at the time of admission and 97 per cent after 24 h, and a negative predictive value of approximately 98 per cent. In addition to requiring 48 h for evaluation, the usual prognostic systems show a sensitivity of 77-85 per cent, a specificity of 70-77 per cent, a positive predictive value of 40-48 per cent, and a negative predictive value of 92-95 per cent, clearly lower than those obtained with leucocyte elastase. Polymorphonuclear elastase is therefore a very early and reliable marker in the diagnosis of the severity of acute pancreatitis, in addition to being easily adaptable to the routine of any hospital laboratory.