The graft-v-host reaction (GVHR), a major complication of bone marrow transplantation, is a complex immunologic phenomenon with a diverse and often confounding presentation. After reviewing major systemic manifestations, this report details the clinical and pathological oral findings of chronic GVHR. The reaction is characterized clinically by a mucosal lichenoid keratosis. Pathological findings resemble mucosal lichen planus, and minor salivary gland show lesions similar to those of Sjögren's syndrome. These oral findings are potentially valuable in the early recognition and follow-up of chronic GVHR. The differential diagnosis includes viral infections and chemotherapeutic drug toxicity, but these can usually be eliminated from consideration by viral cultures and temporal correlation of symptoms, respectively.