The effect of early treatment of cytomegalovirus (CMV) antigenemia with ganciclovir (DHPG) for the prevention of CMV disease was evaluated in 25 patients with hematological malignancy who had received marrow from human leukocyte antigen-matched unrelated donors at our institution. CMV antigenemia occurred in 17 of 25 patients, a high rate of 68%, and was initially detected between 4 and 8 weeks after bone marrow transplantation (BMT) (median time 5 weeks). The incidence of CMV antigenemia was statistically higher in those patients given steroids or antithymocyte globulin (P < 0.01). All 17 CMV antigenemia-positive patients were treated with DHPG until antigenemia was cleared, and this treatment was resumed if antigenemia occurred. CMV antigenemia eventually became negative in all cases. One major drawback of DHPG was that leukopenia was observed in six patients (35%). CMV disease occurred in six patients despite early treatment of CMV antigenemia with DHPG (24%, 6 of 25 patients). Four of them developed CMV disease in the early phase (within six months) and two in the late phase (more than six months). All CMV diseases in the early phase were easily cured by treatment with DHPG while monitoring CMV antigenemia, but CMV disease in the late phase did not respond to DHPG and led to the death of one patient. On the other hand, there were no patients who developed CMV disease in the antigenemia-negative group. Thus, although early treatment using our method was effective in clearing CMV antigenemia in unrelated BMT, it did not totally prevent CMV disease. A further method of early treatment for the prevention of fatal CMV disease is required.