We analyzed previously reported oral letermovir plasma concentration measurements from 40 allogeneic hematopoietic cell transplant (HCT) recipients using model-based pharmacometrics analysis. This analysis highlighted that the industry-sponsored phase III study model significantly over-predicted observed letermovir concentrations. Covariates associated with increased oral clearance (CL/F), possibly contributing to this observation, included: higher weight, younger age, and early use after HCT. The clinical significance and mechanistic explanation of relatively lower real-world exposure remains to be elucidated.