Autoimmune phenomena, most frequently autoimmune hemolytic anemia, is a well-known complication of lymphoproliferative diseases. We report a very rare association of a chronic lymphocytic leukemia with an acquired factor XI inhibitor. A 87-year-old man presented with auto-immune hemolytic anemia. He had untreated chronic lymphocytic leukemia for the past three years and renal insufficiency. Before surgical procedure for arteriovenous fistula, we discovered a very prolonged activated partial thromboplastin time (APTT), and an acquired factor XI inhibitor was detected. The patient was successfully treated with immunosuppressive therapy. Among patients with lymphoproliferative disorders the discovery of a prolonged APTT implies to search for rare autoimmune phenomena like acquired coagulation factor inhibitors.