Acquired coagulation factor deficiencies are caused by inhibitory autoantibodies which are usually directed against clotting factor VIII (FVIII), causing acquired hemophilia A (AHA). Clotting factor inhibitors usually cause abnormalities in screening coagulation tests (activated partial thromboplastin time [aPTT] and/or prothrombin time [PT]). Other coagulation factor inhibitors are much rarer, particularly inhibitors to factor XI (FXI). We present the case of an 82-year-old woman referred to a hematological center for isolated aPTT prolongation in pre-surgery screening tests. No bleeding symptoms were reported either at admission or in the patient’s medical history. One stage coagulation factor assays revealed lower factor VIII, IX, XI, XII levels. The Nijmegen modification of the Bethesda assay showed the presence of an inhibitor to factor XI (22.1 BU/mL). No autoantibodies to coagulation factors VIII, IX and XII were found: inhibitor titers were all below 0.6 BU/mL. Acquired hemophilia C was diagnosed (the presence of autoantibodies to clotting factor XI).