We examined the incidence of herpes varicella-zoster virus (VZV) infection in 151 patients undergoing allogeneic BMT between August 1990 and September 1997 and who survived at least 3 months. Median follow-up was 17 (range 3.3-80.7) months. Herpes simplex virus antibody positive (HSV+) patients received aciclovir 1200 mg p.o. daily or 750 mg i.v. daily, in divided doses from day 0 to engraftment. Ganciclovir (5 mg/kg i.v. three times per week) was given in CMV+ patients (or if the donor was CMV+) from engraftment to day 84. Ganciclovir was continued or recommenced if a dose of greater than 20 mg of prednisone was used for the treatment of GVHD otherwise aciclovir was recommenced. In HSV+ patients not receiving ganciclovir, aciclovir 600 mg p.o. daily in divided doses was given until at least 6 months after BMT. Thirty-two patients developed VZV infection from 4.1 to 28 months after transplant. The estimated cumulative incidence of VZV was 13% (95% confidence interval 6-19%) at 12 months, 32% (22-42%) at 24 months and 38% (27-50%) at 28 months, with no further cases beyond that time. No patient developed VZV whilst receiving aciclovir or ganciclovir (P < 0.0001). However, there was a rapid onset of VZV following cessation of antiviral therapy (33% (20-46%) at 1 year post cessation). The presence of GVHD and the prior duration of antiviral prophylaxis were significant and independent risk factors for the development of VZV. Age, underlying disease, conditioning therapy or donor type were not. We conclude that 3-6 months of low-dose aciclovir and ganciclovir are effective at delaying the onset of VZV after allogeneic BMT, but may not affect the overall incidence of infection. Prolonged prophylaxis may be warranted in patients at high risk of infection, for example those patients with GVHD.