Cytomegalovirus (CMV) infection causes high morbidity and mortality among allogeneic stem cell transplant recipients. Preemptive therapy with oral valganciclovir or intravenous ganciclovir has replaced universal prophylaxis. We prospectively studied 19 consecutive adult recipients of allogeneic peripheral blood stem cell transplants from May 2005 through February 2007 to analyze the safety and efficacy of preemptive therapy for the treatment of CMV infection. The antigenemia test was persistently negative in 8 patients (42%) and positive at least once in 11 (58%). Eight patients were treated with oral valganciclovir on an outpatient basis and they all became CMV negative after the first week of treatment. The other 3 patients received intravenous ganciclovir and were also CMV negative after the first week of treatment. No patient abandoned treatment, no severe secondary toxicity was noted, and there was no CMV-associated mortality.