Both early cytomegalovirus (CMV) monitoring and prophylactic antiviral therapy can decrease clinical complications or can prevent them in patients after allogeneic hematopoietic stem cell transplantation (HSCT). Presented paper summarizes experiences with using regular monitoring of reactivation of CMV after allogeneic HSCT by qualitative polymerase chain reaction (PCR) method to prevent the development of symptomatic CMV disease. Samples of peripheral blood leukocytes (PBL) in 71 patients were monitored. Because of retransplantation in two patients, 73 transplantations, each followed by the monitoring, were performed. Patients were monitored weekly after the transplantation for CMV DNA-emia in PBL. An episode of CMV infection representing an indication for preemptive ganciclovir (GCV) or foscarnet (FOS) therapy was defined as two consecutive positive PCR results in 4-7 days. Median time of monitoring was 313 days. The CMV infection was found in 28/73 monitorings (38.4%) and always was followed by preemptive therapy. One recurrence of CMV infection was observed in 4/28 (14.3%) monitorings and two recurrences in 1/28 (3.6%) monitorings. Presented approach resulted in complete prevention of overt CMV disease and this study enable to show that qualitative PCR method for determination of incipient CMV infection followed by preemptive therapy is suitable for preventing patients after allogeneic transplantation from CMV disease.