The aim of this study was to assess the incidence of cytomegalovirus (CMV) infection and disease in patients with hematologic malignancies treated with alemtuzumab. The outcome of CMV infection in hematologic patients treated with alemtuzumab in 19 hospitals throughout Spain was assessed retrospectively. Data were collected from the medical records of patients over a period of 6 months following initiation of alemtuzumab therapy. We studied 102 patients (89 with B-cell chronic lymphocytic leukemia and 13 with other lymphoproliferative diseases, with a median age of 63 years [range 29-81 years]). Alemtuzumab was administered for a mean of 11.2 (standard deviation: 13.8) weeks, with a median total dose of 423 mg (range: 59-1440 mg). Alemtuzumab as a single agent was administered in 92.2% of patients and was associated with chemotherapy in 7.8% of cases. Prophylactic antivirals included famcyclovir (47%), acyclovir (34%), valacyclovir (14%) and valgancyclovir (5%). CMV viremia testing was performed a mean of 6.3 times (range: 1-19). The incidence of CMV infection was 38.9% (46% in patients treated with steroids and 75% in patients receiving ≥1000 mg of alemtuzumab). Treatment of CMV infection included gancyclovir or valgancyclovir in 94% of cases. Viremia became negative after a median of 20 days (95% CI: 13.4-26.6). CMV disease occurred in five patients. The incidence of CMV infection in alemtuzumab-treated patients was 38.9%. The incidence increased in patients treated concomitantly with steroids and in those treated with high doses of alemtuzumab, although only eight patients received 1000 mg or more, systematic monitoring of CMV viremia and early treatment of infection resulted in a favorable outcome of CMV reactivation.