As part of a European multicenter study to investigate the potency of FK506 in primary immunosuppression after liver transplantation, this comparison with our conventional cyclosporine-based quadruple regimen was carried out as a controlled, prospective, randomized trial. The 121 patients entering the study were randomly assigned to receive immunosuppressive regimens consisting either of FK506 and prednisolone (FK/n = 61) or of cyclosporine, prednisolone, azathioprine, and a 7-day course of rabbit antithymocyte globulin (CsA/n = 60). Rejection was suspected in the case of scant production of light bile or biochemical graft dysfunction, without evidence of vascular, biliary, or infectious complications. A liver biopsy for confirmation of the diagnosis was obtained each time. Initial therapy entailed a 3-d course of high-dose methyl-prednisolone. Steroid resistant rejections were treated with OKT3 monoclonal antibody or, in the group of primary CsA administration, conversion to FK506 as another treatment option. One-year patient (FK: 90.2%; CsA: 96.7%) and graft survival (FK: 88.5%; CsA: 91.7%) did not differ significantly. Overall, 41 patients (33.9%) experienced 50 acute, cellular rejection episodes (RE) [FK: 25 RE in 21 patients (34.4%); CsA: 25 RE in 20 patients (33.3%)]. The histological grading ranged from mild (FK: 14/25; CsA: 8/25) to moderate (FK: 9/25; CsA: 16/25) and severe (FK: 2/25; CsA: 1/25): not significantly different between the two groups. In the CsA-based group, three additional rejection episodes were classified as early chronic (n = 1) and chronic rejection (n = 2).(ABSTRACT TRUNCATED AT 250 WORDS)