Secondary cancers developing after allogeneic hematopoietic stem cell transplantation generally originate from recipient-derived cells. In this study, we analyzed the tumor cell origin of 5 epithelial malignant tumors (esophageal squamous cell carcinoma, lung adenocarcinoma, gastric adenocarcinoma, pharyngeal squamous cell carcinoma, and thyroid papillary carcinoma) that developed after allogeneic peripheral blood stem cell transplantation using anti-AE1/3 immunofluorescence with fluorescence in situ hybridization analysis for sex chromosomes and/or short-tandem repeat microsatellite analysis of laser-microdissected tumor cells. The results revealed that 1 of these 5 cancers was derived from donor cells. In this case, transfused pluripotent cells, which include both mesenchymal stem cells and hematopoietic stem cells, might have given rise to epithelial malignant cells. Our observations suggest that transfused peripheral blood cells may be involved in the development of cancers after allogeneic peripheral blood stem cell transplantation.