This article describes a gingival squamous cell carcinoma that developed in a 21-year-old woman who received a bone marrow transplant at the age of 16 from her human leukocyte antigen-identical sister as treatment for severe aplastic anemia. Thirty days after transplantation, she presented with cutaneous erythema as a result of acute graft-versus-host disease, and this subsequently evolved into chronic graft-versus-host disease. A lichenoid white plaque of the gingiva developed shortly thereafter, and it began to increase in size rapidly 4 years posttransplantation. Biopsy indicated squamous cell carcinoma arising in this region, apparently associated with chronic graft-versus-host disease. Few reports have described a secondary solid malignancy involving the oral cavity of young adults after bone marrow transplantation.