Human cytomegalovirus (HCMV) can cause significant disease in immunocompromised patients, and treatment options are limited by toxicities. CSJ148 is a combination of two anti-HCMV human monoclonal antibodies (LJP538 and LJP539) that bind to and inhibit the functions of viral HCMV glycoprotein B (gB) and the pentameric complex, consisting of glycoproteins gH, gL, UL128, UL130, and UL131. In this phase 2, randomized, placebo-controlled trial, we evaluated the safety and efficacy of CSJ148 for prophylaxis of HCMV in patients undergoing allogeneic hematopoietic stem cell transplantation. As would be expected in the study population, all the patients (100%) reported at least one treatment-emergent adverse event. There were 22 deaths during this study, and over 80% of the patients receiving placebo or CSJ148 developed at least one adverse event of grade 3 or higher severity. No subject who received antibody developed a hypersensitivity- or infusion-related reaction. CSJ148-treated patients showed trends toward decreased viral load, shorter median duration of preemptive therapy, and fewer courses of preemptive therapy. However, the estimated probability that CSJ148 decreases the need for preemptive therapy compared to placebo was 69%, with a risk ratio of 0.89 and a 90% credible interval of 0.61 to 1.31. The primary efficacy endpoint was therefore not met, indicating that CSJ148 did not prevent clinically significant HCMV reactivation in recipients of allogeneic hematopoietic cell transplants. (This study has been registered at ClinicalTrials.gov under identifier NCT02268526 and at EudraCT under number 2017-002047-15.)