Twenty patients undergoing allogeneic bone marrow transplantation and 39 patients receiving remission induction chemotherapy for acute leukaemia were entered into a double blind, placebo controlled stratified trial of acyclovir prophylaxis against herpes group virus infections. Within the transplant group intravenous acyclovir 5 mg/kg twice daily given throughout the period of granulocytopenia completely prevented oropharyngeal herpes simplex virus infection compared with a 50% incidence in the placebo arm (p = 0.033). The acyclovir group also had fewer days of fever during the trial and a shorter duration of leukopenia, possibly because of the prevention of herpes simplex virus infections. There was a high incidence of herpes infections after the trial in patients who received either acyclovir or placebo. In the non-transplant group there was also a significant reduction of herpes simplex virus infection in the oropharynx and oesophagus (two out of 19 patients as compared with 10 out of 20; p = 0.018). Herpes simplex virus was isolated in the acyclovir arm within a day after starting the trial in one patient, and the other failure was due to a virus with reduced sensitivity to acyclovir in a patient who had had several previous courses of the drug. The incidence of herpes infections after stopping treatment was low. The influence of acyclovir on excretion of Epstein-Barr virus in saliva in either group was inconclusive. One patient (transplant group) developed a cytomegalovirus infection while receiving acyclovir. Acyclovir provides effective prophylaxis against oropharyngeal and oesophageal herpes simplex virus infection in severely immunocompromised seropositive (greater than or equal to 1/8) patients. In patients given bone marrow transplants this may have the additional benefit of reducing the time to recovery of an adequate blood count and the number of days of fever.