BACKGROUND: Calcineurin inhibitors are the most commonly used immunosuppressive drugs in liver transplantation, but the optimum initial immunosuppression regimen is not known. The aim of our study was to compare tacrolimus with microemulsified ciclosporin, in a regimen with standardised concomitant immunosuppressive therapy. METHODS: In all liver transplant centres in the UK and Republic of Ireland, 606 patients undergoing a first orthotopic liver transplantation were randomly assigned open-label tacrolimus or microemulsified ciclosporin. Primary outcome was the combined frequency (whichever occurred first) of death, retransplantation, or treatment failure for immunological reasons, analysed by intention to treat. FINDINGS: 96% of patients received the treatment allocated to them. The primary outcome was reached in 62 (21%) of 301 patients in the tacrolimus group versus 99 (32%) of 305 allocated microemulsified ciclosporin (relative risk 0.63 [95% CI 0.48-0.84], p=0.001; time-to-event analysis log-rank test p=0.002): deaths (50 [17%] vs 72 [24%]); retransplantations (11 [4%] vs 31 [10%]) treatment failure for immunological reasons (6 [2%] vs 12 [4%]). The relative risk for the composite outcome was in favour of tacrolimus. The main causes of death in both trial groups were sepsis and multiple organ failure (31 [10%] vs 30 [10%]), and the main cause for retransplantation was hepatic artery thrombosis (6 [2%] vs 17 [6%]). Renal dysfunction and the need for antihypertensive therapy were much the same in both groups. Tacrolimus was more diabetogenic. INTERPRETATION: Clinical outcome at 1 year was better with tacrolimus-based immunosuppression than with microemulsified ciclosporin during the first year after liver transplantation. Tacrolimus should be the first choice of calcineurin inhibitor for patients receiving their first liver graft.