Cytomegalovirus (CMV) infection is one of the most common causes of morbidity and mortality after allogeneic marrow transplantation. We studied 14 consecutive CMV-seropositive patients adding ganciclovir (2.5 mg/kg i.v. every 8 hours for 7 days prior to transplant and 6 mg/kg three times a week after neutrophils became greater than 0.5 x 10(9)/l and the patients were platelet transfusion-independent until d 70) to our previous prophylaxis regimen which consisted of intravenous immunoglobulin and acyclovir. The result was compared with 30 consecutive patients whom we studied with our previous regimen. The addition of ganciclovir did not cause any extra toxicities. The incidence of interstitial pneumonitis and cumulative probability of CMV excretion in the first 100 d post-transplantation was significantly reduced (p = 0.038 and p = 0.035 respectively). The result shows that addition of ganciclovir significantly decreased the incidence of CMV infection in the early post-transplantation period.