A frequent complication of anticancer treatment, oral and gastrointestinal (GI) mucositis, threatens the effectiveness of therapy because it leads to dose reductions, increases healthcare costs, and impairs patients' quality of life. The Multinational Association of Supportive Care in Cancer and the International Society for Oral Oncology assembled an international multidisciplinary panel of experts to create clinical practice guidelines for the prevention, evaluation, and treatment of mucositis. The panelists examined medical literature published from January 1966 through May 2002, presented their findings at two separate conferences, and then created a writing committee that produced two articles: the current study and another that codifies the clinical implications of the panel's findings in practice guidelines. New evidence supports the view that oral mucositis is a complex process involving all the tissues and cellular elements of the mucosa. Other findings suggest that some aspects of mucositis risk may be determined genetically. GI proapoptotic and antiapoptotic gene levels change along the GI tract, perhaps explaining differences in the frequency with which mucositis occurs at different sites. Studies of mucositis incidence in clinical trials by quality and using meta-analysis techniques produced estimates of incidence that are presented herein for what to our knowledge may be a broader range of cancers than ever presented before. Understanding the pathobiology of mucositis, its incidence, and scoring are essential for progress in research and care directed at this common side-effect of anticancer therapies. Copyright 2004 American Cancer Society.