[Infiltrating mucinous carcinoma developed on chronic anal fistula; diagnostic and surgical difficulties].

Anal carcinoma is rare; fistula-in-ano is sometimes mentioned among other premalignant conditions. Chronic anal fistula is associated with malignant transformation to mucinous adenocarcinoma, difficult to diagnose in the anal region and associated to a poor prognosis. Malignant transformation to infiltrating colloid carcinoma was surprised during a one year period, in a patient with long-standing perianal fistula (24 years).