In this prospective single-center study, we evaluated the efficacy and safety of valaciclovir (VACV) in the prevention of cytomegalovirus (CMV) infection after allogeneic bone marrow transplantation (BMT). The study population consisted of 12 patients who underwent allogeneic BMT from an unrelated donor. Patients received acyclovir (ACV) intravenously until they became able to take VACV orally. VACV was administered at a daily dose of 3000 mg and continued until day 100. CMV infection was monitored by CMV antigenemia assay and real-time polymerase chain reaction analysis of plasma. Thirty-five patients who did not receive any form of CMV chemoprophylaxis served as control subjects. CMV infection was detected in 4 (33.3%) of the 12 patients and in 24 (68.6%) of the 35 control subjects (P < .05). The onset of CMV infection was significantly delayed in the VACV group (median, day 43) compared with the control group (median, day 28.5; P < .01). Gastrointestinal symptoms as an adverse event due to VACV administration were observed in 2 patients. The plasma levels of ACV after VACV administration were measured in 8 patients and were similar to those in the healthy subjects. In conclusion, VACV shows normal absorption, even in the early posttransplantation period, and may prevent or delay CMV infection effectively and safely in allogeneic BMT recipients.