Recipients of HLA-matched, related or unrelated allogeneic BMT who were CMV seropositive or those receiving unmanipulated marrow from a seropositive donor were randomised to receive one of three treatment regimens, i.v. acyclovir 500 mg/m2 three times a day from 5 days before transplant to 30 days after transplant followed by oral acyclovir 800 mg four times a day for a further 6 months, i.v. acyclovir followed by placebo, or 400 mg oral acyclovir four times a day followed by placebo (control). This paper reports the 1 year data on the same cohort of patients which was previously reported. Intravenous acyclovir (i.v./PCB) significantly reduced the risk of CMV infection when compared to the control group. The frequency of adverse events reported was comparable among the three groups. The mortality rate was significantly reduced by the sequential use of i.v. acyclovir followed by oral acyclovir, resulting in a 19% survival advantage at 1 year from transplant.