Cytomegalovirus (CMV) infection is a major cause of morbidity and mortality after allogeneic bone marrow transplantation (BMT). Our aim was to study the prophylactic effect of high-dose intravenous acyclovir given around the time of BMT followed by oral acyclovir on CMV infection and survival. 310 BMT recipients at risk of developing CMV infection were randomised to one of three regimens in a double-blind and double-dummy design: intravenous acylclovir (500 mg/m2, three times a day) for 1 month followed by oral acyclovir (800 mg four times a day for a further 6 months) (intravenous/oral group); intravenous acyclovir followed by oral placebo (intermediate group); or low-dose oral acyclovir (200 or 400 mg, four times a day) followed by placebo ("controls"). Analysis was by intention-to-treat. Intravenous acyclovir significantly reduced the probability of and delayed the onset of CMV infection. There was no further reduction in infection risk with the addition of long-term oral acyclovir. Time to CMV viraemia was delayed in the intravenous/oral acyclovir group compared with controls. Extending the prophylaxis with oral acyclovir significantly improved survival: 79 of 105 recipients were still alive at 7 months compared with 60 of 102 controls (p = 0.012). Although the intravenous/oral acyclovir group did significantly better than controls in terms of survival, the difference between the intravenous/oral acyclovir group and the intermediate group was of borderline statistical significance (p = 0.054). Adverse events that were possibly treatment related were similar in all three groups. The most commonly reported events were nausea, vomiting, elevated creatinine, and renal failure. High-dose intravenous followed by oral acyclovir improved survival and was of benefit in prophylaxis against the effects of CMV after BMT. Interpretation of CMV infection was made difficult because an intermediate treatment (intravenous acyclovir followed by oral placebo) was as effective as high-dose intravenous/oral acyclovir.