Abstract: Cord blood stem cell transplantation is an important alternative for patients needing hematopoietic stem cell transplantation. However, it is unclear how cord blood cells, which are 0-year-old, age in the recipient’s body after allogeneic transplantation. We performed DNA methylation (DNAm) age analysis to measure the age of cells using post-transplant peripheral blood in 50 cases of cord blood transplantation. The median chronological age (the time elapsed from the date of the cord blood transplant to the day the sample was taken for DNAm analysis) of donor cells was 4.0 years (0.2 – 15.0 years), while the median DNAm age was 10.0 years (1.3 – 30.3 years), and the ratio of DNAm age to chronological age (AgeAccel) was 2.7 (1.2 – 8.2). When comparing the mean values of AgeAccel in cord blood transplant cases and controls, the values were significantly higher in cord blood transplant cases. The characteristics of patients and transplant procedures were not associated with AgeAccel in this analysis, nor were they associated with the development of graft-versus-host disease. However, this analysis revealed that transplanting 0-year-old cord blood into a recipient resulted in cells aging more than twice as quickly as the elapsed time. The results shed light on the importance of the mismatch between cord blood stem cells and donor environmental factors in stem cell aging.