Studies in the USA and Japan have shown that tacrolimus (FK506) is a potent immunosuppressant. To compare the efficacy and safety of tacrolimus-based and conventional cyclosporin-based immunosuppressive regimens we recruited 545 liver transplant recipients from eight European centres into a randomised open trial. Analysis of the data at 12 months post-transplant showed that tacrolimus was associated with a significant reduction in acute, refractory acute, and chronic rejection episodes. The rates were, for acute rejection, tacrolimus 40.5% vs 49.8% cyclosporin (p = 0.040; absolute difference 9.3% [95% CI 0.9-17.8%]). For refractory acute and chronic rejections the comparisons were 0.8% vs 5.3% (p = 0.005) and 1.5% vs 5.3% (p = 0.032). There results were seen despite significantly lower corticosteroid usage. The incidence of infection was also lower in patients receiving tacrolimus. Patient and graft survival rates were not significantly different (tacrolimus 82.9% and 77.5%; cyclosporin 77.5% and 72.6%). Safety data were comparable--the most serious events being renal impairment, disturbances of glucose metabolism, and neurological complications--but these events were more common in the tacrolimus group. In this trial tacrolimus had advantages over cyclosporin in respect of lower rejection rates, even though less concurrent immunosuppression was administered.