Allogenic peripheral stem cell transplantation (HSCT), a procedure that is widely used in the treatment of a large number of malignant and nonmalignant hematological diseases, is still associated with a wide range of complications, one of the most important of which is graft-versus-host disease (GVHD). The patients undergoing allogeneic HSCT are at high risk of developing secondary neoplasms, particularly leukemias and lymphomas. Solid tumors are less frequent, but their incidence seems to be higher in the patients who develop GVHD; the most frequent solid tumors are squamous cell carcinomas. We here describe the clinical course and histopathologic aspects of a squamous cell carcinoma arising on GVHD-induced oral lesions in a 53-year-old woman with non-Hodgkin's lymphoma undergoing allogeneic HSCT. Immediately after the transplantation, the patient developed GVHD involving the gastroenteric tract, skin, joints, and oral cavity, which was treated with cyclosporin, prednisone, azathioprine, colchicine, and photophereses. In addition to the sporadic reports of similar pictures published in the literature (16 cases of squamous cell carcinoma owing to oral GVHD in patients undergoing allogeneic HSCT), our case underlines the susceptibility of HSCT patients with oral GVHD to carcinoma of the oral cavity. All patients treated with allogeneic HSCT (particularly those who have developed GVHD) should therefore undergo a careful examination of the oral mucosa and be closely followed up over the long term with the aim of identifying the onset of secondary tumors as early as possible.