A middle-aged man sought medical attention because of a large perianal mass. The lesion was caused by a massive subepithelial infiltrate of small neoplastic lymphocytes representing a localized tissue manifestation of chronic lymphocytic leukemia. This report emphasizes the possibility of leukemia/lymphomas occurring at the anus, to stress that leukemia should be considered in the differential diagnosis of patients presenting with perianal disease in the presence of lymphadenopathy and constitutional symptoms.