Alemtuzumab is an immunosuppressive antibody that depletes normal T cells and B cells. Prophylaxis for herpes virus and Pneumocystis carinii is standard with this agent. Approximately 20% to 25% of patients will experience cytomegalovirus (CMV) reactivation. We conducted a randomized trial wherein patients being treated with an alemtuzumab-containing regimen received prophylaxis with either valaciclovir 500 mg orally daily or valganciclovir 450 mg orally twice daily. The study design planned to enroll 128 patients, but stopping rules for early termination were met. Forty patients were evaluable. Median age was 58 years (range, 25-83 years); median number of prior therapies was 2 (range, 0-10). Diagnoses included chronic lymphocytic leukemia (29), T-cell prolymphocytic leukemia (3), hairy cell leukemia (1), adult T-cell leukemia/lymphoma (ATLL) (1), marginal zone leukemia (1), large granular lymphocyte leukemia (2), acute lymphoblastic leukemia (1), and T-cell lymphoma (2). Patients received various alemtuzumab-containing regimens, including single agent (5) or combined with: rituximab (2), pentostatin (6), fludarabine, cyclophosphamide, and rituximab (23), or fractionated cyclophosphamide, vincristine, adriamycin, and dexamethasone (hyper-CVAD) (4). Seven of 20 patients enrolled on the valaciclovir arm experienced CMV reactivation. None of the 20 patients randomized to valganciclovir experienced CMV reactivation (P = .004). In conclusion, this agent was highly effective for prophylaxis of CMV reactivation in patients receiving alemtuzumab. This trial was registered at www.ClinicalTrials.gov as #NCT00562770.