Two consecutive studies are reported. In study 1, 59 patients with acute leukaemia undergoing remission induction therapy or bone marrow transplantation were given either acyclovir (5 mg/kg) or placebo by intravenous infusion twice daily during the period of neutropenia. All patients were seropositive (⋝ 1: 8) for herpes simplex virus. Acyclovir provided total protection against herpes simplex virus infection in bone marrow transplant patients with 0/10 versus 5/10 infections in the placebo group ( P = 0.033). For the remission induction therapy group 2/19 on acyclovir developed HSV versus 10/20 receiving placebo ( P = 0.018). Only one episode of cytomegalovirus (CMV) infection was seen and this was in a patient on acyclovir. Epstein-Barr virus secretion studies were inconclusive. No varicella zoster virus (VZV) infections were seen. In Study 2, 20 consecutive HSV seropositive (⋝ 1: 8) bone marrow transplant recipients received oral acyclovir. Five (25%) developed HSV infections, one of which was in a non-compliant patient, who subsequently developed a fatal infection whilst receiving therapeutic (5 mg/kg 8 hourly) intravenous acyclovir for this HSV infection. Intravenous acyclovir is effective in preventing HSV infections in the immuno-compromised host but it seems unlikely that CMV will be amenable to acyclovir in its present formulation.