Our objective in this study was to determine the efficacy of 2 grams a day of oral acyclovir administered for 16 weeks after transplantation for the prevention of cytomegalovirus (CMV) infection and disease in CMV-seropositive liver transplant recipients. Seventy-three adult liver transplant recipients, seropositive for CMV, were randomized to receive either 2 grams a day of oral acyclovir for 16 weeks after transplantation or no prophylaxis. The incidence of CMV disease was significantly lower in the acyclovir group (5%) than in the control group (27%; P < 0.05). By log-rank analysis, the differences in the probability of presenting CMV disease over the first 16 weeks and over the 1st year were also significant (P < 0.05). We conclude that 2 grams a day of oral acyclovir provides effective prophylaxis against CMV disease in CMV-seropositive liver transplant recipients.