Mucositis induced by chemotherapy is a painful and often dose-limiting side effect of cancer therapy. Furthermore, loss of the integrity of the oral epithelium often provides a microbial portal of entry and leads to sepsis. The present study describes the first animal model for chemotherapy-induced mucositis. The combination of three intraperitoneal injections of 5-fluorouracil at 5-day intervals and superficial mechanical mucosal irritation resulted in clinical breakdown of the oral mucosa characterized by ulcerative mucositis in Golden Syrian hamsters. Both clinical and histologic evaluation demonstrated that these changes were similar to those described in human beings and followed a pattern influenced by the degree of myelosuppression. This model should be of significance in establishing the stomatotoxicity of new chemotherapeutic agents, in evaluating medicaments to treat mucositis, and in studying the influence of oral mucosal breakdown on sepsis in myelosuppressed persons.