Bone marrow transplantations are being used with increasing frequency in the treatment of patients with leukemia and aplastic anemia. The graft-vs.-host disease (GVHD) is a serious complication that affects long-term survivors following bone marrow transplantation. It is the result of an immunologic reaction mounted by the grafted reticuloendothelial cells against the tissues of the recipient, and it affects multiple organ systems. Involvement of the skin and mucosal surfaces of the head and neck region, in particular the oral cavity, occurs in a large number of patients with GVHD. In this report we present four patients with GVHD in whom mucosal lesions and infections of the head and neck region were prominent features. Our observations indicate that the clinical and histological characteristics of these lesions vary according to the time elapsed from the onset of the disease. Therefore, clinical examination of the head and neck region and biopsy of the oral mucosa are important not only in the diagnosis of the GVHD, but also in the evaluation of its progress and response to treatment.