BACKGROUND: Cytomegalovirus (CMV) infection is common following thoracic organ transplantation and causes substantial morbidity and mortality. Letermovir is a novel antiviral agent used off-label in this population for CMV prevention. Our goal was to understand patterns of letermovir use and effectiveness when applied for CMV prophylaxis after thoracic transplantation. METHODS: We retrospectively evaluated letermovir use among thoracic transplant recipients at an academic transplant center who initiated letermovir from January 2018 to October2019 for CMV prophylaxis. We analyzed indication, timing, and duration of prophylaxis; tolerability; and occurrence of breakthrough CMV DNAemia and disease. RESULTS: Forty-two episodes of letermovir prophylaxis occurred in 41 patients, including 37 lung and 4 heart transplant recipients. Primary prophylaxis (26/42, 61.9%) was utilized mainly due to myelosuppression (25/26, 96.2%) and was initiated a median of 315 days post-transplant (interquartile range [IQR] 125-1139 days). Sixteen episodes of secondary prophylaxis (16/42, 38.1%) were initiated a median of 695 days post-transplant (IQR 537-1156 days) due to myelosuppression (10/16, 62.5%) or prior CMV resistance (6/16, 37.5%). Median duration of letermovir prophylaxis was 282 days (IQR 131-433 days). Adverse effects required letermovir cessation in 5/42 (11.9%) episodes. Only one episode (2.4%) was complicated by clinically significant breakthrough CMV infection. Transient low-level CMV DNAemia (<450 IU/ml) occurred in 15 episodes (35.7%) but did not require letermovir cessation. CONCLUSIONS: Letermovir was well tolerated and effective during extended prophylactic courses with only one case of breakthrough CMV infection in this cohort of thoracic transplant recipients. Further prospective trials of letermovir prophylaxis in this population are warranted.