Oral complications of cancer therapy often have systemic consequences. Pain and discomfort are common and can lower intake of fluid and nutrients, which in severe cases can lead to dehydration and malnutrition, requiring hospitalization. Oral infections are frequent accompaniments of cancer treatment. Herpes simplex virus is the most common symptomatic oral viral infection, and, in latently infected patients the virus is frequently reactivated after cytoreductive therapy. Viral (infectious) oral mucositis is often indistinguishable from noninfectious mucositis. Bacterial infections are less commonly observed today, perhaps because of the routine use of empiric broad-spectrum antibiotics; however, many episodes of septicemia in neutropenic patients apparently originate from oral microorganisms. Fungal infections are frequent and are usually due to Candida species. Spread to the esophagus or systemic dissemination can occur. Noninfectious oral mucositis can be used as a marker of toxic effects in other organs, especially hepatic veno-occlusive disease. In bone marrow transplant patients with mucositis, hepatic veno-occlusive disease is six times more frequent than in such patients without mucositis.