Two cases of secondary oral squamous cell carcinoma (SCC), which developed in recipients of hematopoietic stem cell transplantation (HSCT) for leukemia, are reported. The first patient underwent allogeneic HSCT for chronic myelogenous leukemia at 32 years of age. He suffered from chronic graft-versus-host-disease (GVHD) of the oral mucosa after HSCT, and has subsequently received immunosuppressive therapy. He experienced metachronous multiple SCCs in the maxillary gingiva and the dorsum of the tongue at 36 years and 40 years of age, respectively. The second patient received autologous HSCT for acute myelogenous leukemia at 22 years of age, and she did not experience GVHD after transplantation. SCC developed in the lateral border of the tongue at 27 years of age. PCR analysis detected both HPV16 and HPV18 in the tongue tumor of the first patient, and only HPV18 in that of the second patient, suggesting that the infection of high-risk HPVs was possibly involved in the development of post-HSCT oral cancers in these patients. Since risk factors for post-HSCT oral SCC are not yet well recognized, long-term close follow-up is necessary for the early detection of secondary oral cancers in all transplant recipients.