Despite controversial experimental and clinical findings, the use of Aprotinin (A) in the treatment of acute pancreatitis (AP) is still widespread in our country. Compared to A, Gabexate Mesilate (FOY) should have the following advantages: lack of antigenicity, low molecular weight with better cellular penetrance, wide inhibitory spectrum (against phospholipase A2 too) and good tolerability. In order to verify the efficacy of FOY versus, we decided to carry out a prospective multicenter (34 centers) randomized double blind trial in moderate-severe AP with FOY 3g/day versus A 1,500,000 U.I.K./day infused for 7 days with central venous line. Based on standard clinical, biochemical and radiological data, 199 patients suffering from AP were enrolled to the study from January 1989 to December 1990. The main admission criteria were: onset of the disease from no more than 72 hours, at least 2 Ranson positive criteria and C.T. evidence of acute pancreatic damage. Seventeen no protocol patients were excluded. The Etiology in the 182 analyzed patients (106 male, 76 female; mean age 57.6 years, range 21-91) were: biliary 102 (56%), alcohol 47 (25.8%), post-operative 13 (7.1%), unknown causes 13 (7.1%) and others 7 (3.8%). Median Ranson's Score was 3. CT examination showed 66 AP with edema, while 116 cases (63.7%) were recognized as severe forms with pancreatic necrotic involvement of 30% of the gland in 71 (61.2%), 50% in 31 (26.7%), and more than 50% in 14 (12.1%). Out of 182 patients, 91 were treated with A and 91 with FOY. The two groups were comparable regarding the main prognostic factors.(ABSTRACT TRUNCATED AT 250 WORDS)