Cytomegalovirus is a major viral pathogen in patients who undergo renal transplantation, and cytomegalovirus disease is difficult to treat. We therefore conducted a randomized, placebo-controlled, double-blind trial of acyclovir for the prevention of cytomegalovirus disease in recipients of renal allografts from cadavers. Acyclovir was given orally in doses of 800 to 3200 mg per day, according to the patients' estimated level of renal function. Patients took the first dose of either acyclovir or placebo six hours before transplantation and continued to take the assigned medication for 12 weeks. Of 118 patients enrolled in the study, 104 completed at least 30 days on the study medication and were included in our analysis of the results. During the first year after transplantation, 4 of 53 patients (7.5 percent) in the acyclovir group had symptomatic cytomegalovirus disease, as compared with 15 of 51 (29 percent) in the placebo group (P = 0.002). There was a single case of cytomegalovirus pneumonia in the acyclovir group, as compared with nine in the placebo group. The greatest prophylactic benefit of acyclovir was observed among seronegative patients who had received a kidney from a seropositive donor; only one of six such patients in the acyclovir group had cytomegalovirus disease, as compared with all seven in the placebo group. Acyclovir decreased the incidence of documented cytomegalovirus infection (with or without symptomatic disease) to 36 percent from 61 percent among the patients who received the placebo (P = 0.011). Among the patients who received acyclovir, the rates of recovery of virus from the blood and urine were significantly reduced, but the rate of viral shedding from the pharynx was not significantly different from that in the placebo group. There were no differences between the groups in the frequency of adverse events or in the rate of survival of either grafts or patients. We conclude that the oral administration of acyclovir, beginning before the transplantation of a renal allograft from a cadaver, reduces the rate of cytomegalovirus infection and disease without affecting the survival rate of either grafts or patients.