The prophylactic effect of acyclovir (ACV) on varicella-zoster virus (VZV) infection in leukaemia patients who have undergone bone marrow transplantation (BMT) was reviewed. The benefits of the use of the laminar air flow (LAF) room in the prevention of nosocomial VZV infections in the haematological ward are also discussed. Since 1986 ACV has been administered to BMT patients to prevent herpes simplex virus (HSV) infections. Of 98 patients with leukaemia who underwent BMT, 73 received ACV (200 mg five times daily) and 25 were not given ACV. In the untreated group, 9 patients (36.0%) developed VZV infection by day 67 (median) and 3 patients died due to disseminated VZV infection. In the ACV-treated group, 18 patients (24.6%) developed VZV infection by day 150 (median) and there were no deaths. From July to December 1989, nine cases of VZV infections (eight patients and one nurse) were reported in the haematological ward of the hospital. All cases originated in the conventionally ventilated areas of the ward while no VZV infections were reported in the 14 patients who occupied the LAF rooms during the same period.