Assessment of the efficacy of therapeutic approaches to anal lesions of Crohn's disease is frustrated by the lack of precise definition of its various manifestations. A classification that is clinical and based on anatomic and pathologic aspects is presented; it has been derived from a 20-year prospective study of anal Crohn's disease in Cardiff. Conceptually, the classification is analogous to the TNM system for cancer. The main classification (U.F.S.) defines the presence of Ulceration, Fistula/abscess, and Stricture, qualified by numeric values reflecting severity (0 = not present, 1 = limited clinical impact, and 2 = severe). A subsidiary classification (A.P.D.) defines Associated conditions, Proximal intestinal involvement, and Disease activity. In addition, the classification may be used in a detailed form for research or comparative purposes or in a simple form defining only the dominant lesions for routine clinical use. General use of the classification would make it possible to compare in detail incidence, management, and results of treatment in different centers.