In this study, we investigated the effects of an Eriobotrya japonica seed extract (ESE) on mucositis using a 5-fluorouracil (5-FU)-induced mucositis hamster model. This model was prepared by intraperitoneally administering 90 mg/kg of 5-FU to hamsters on Day 1, scratching 1 cm(2) of the left cheek pouch of hamsters with a wire brush on Days 2, 3, and 4, and intraperitoneally administering 60 mg/kg of 5-FU on Day 5. Mucositis was evaluated based on the mucositis score at the mucositis site, left cheek pouch thickness, histological findings on HE staining, and plasma lipid peroxide levels. On Day 10, the mucositis score and left cheek pouch thickness in the ESE group were significantly lower than those in the tap water group. Histologically, the two groups showed a defect of the cheek pouch epithelium on Day 6. On Day 10, epithelial injury and bacterial infection were noted in the tap water group. However, in the ESE group, similar findings were not observed. On Day 6, the plasma lipid peroxide level in the tap water group was significantly higher than that in the normal group. In the ESE group, the plasma lipid peroxide level was significantly lower than that in the tap water group. These results suggest that ESE is useful for treating chemotherapy-induced mucositis.