This study was performed to evaluate the incidence, risk factors, and outcome of cytomegalovirus (CMV) infection in autologous stem cell transplantation (ASCT), with the aim of performing preemptive therapy in patients with antigenemia. Starting from 2001, 171 consecutive ASCTs were performed in 136 patients; 102 of these patients were seropositive for CMV at the onset of hematological disease. In all these patients, a CMV pp65 antigenemia assay was determined weekly, starting from the day when the absolute neutrophil count went above 500/microL, and until day 60 after ASCT; subsequently, antigenemia was determined only when a CMV infection was suspected. Among the 136 transplanted patients, 40 (29.4%) presented a positive antigenemia; all of them were seropositive for CMV before ASCT; and no cases of primary infection were seen. The incidence of CMV infection in the seropositive population was 40/102 (39.3%); 6 patients (5 with multiple myeloma and 1 with non-Hodgkin's lymphoma) who received 2 ASCTs developed CMV infections after both transplantations, so that positive antigenemia developed after 46/171 (26.9%) transplantations. First positive antigenemia presented a median of 32 days (range 7-57) after stem cell reinfusion. The median antigenemia level at the first appearance was 2/200,000 (range 1-1000). No significant prognostic factors could be shown. Enteritis was present in 5 patients; 2 of them also had fever, and 1 of them also had thrombocytopenia. In 5 patients fever without any other clinical signs or symptoms was present; 30 patients were asymptomatic. Fourteen patients were treated with anti-CMV drugs. CMV reactivation was successfully treated in all patients, and no patient died from CMV disease.