We report on the rapid elimination of a potent spontaneous factor V antibody of undetermined etiology by extracorporeal immunoadsorption on sepharose-bound polyclonal sheep antibodies to human immunoglobulins (Ig-Therasorb, Baxter) in combination with immunosuppressive treatment. A 68-year-old woman presented with severe hematuria. Severe factor V deficiency (< 1%) caused by an antibody to factor V (26 BU/ml) was found. Extracorporeal immunoadsorption (8.245 +/- 553 ml plasma processed per session) led to an average reduction of the antibody titer by 75% per session. The procedure was well tolerated without any side effects. Hematuria ceased after three immunoadsorptions and complete elimination of the antibody was achieved after seven sessions (day 15), followed by a rapid increase of the factor V activity to normal levels. Treatment with cyclophosphamide and prednisone was started on day 6 and continued for 2 months. The patient remains in remission at 6 months. Extracorporeal immunoadsorption is a highly effective method for eliminating antibodies to factor V (or other clotting factors) in selected cases, i.e., in patients with severe bleeding tendency, high antibody titer, and low probability of a rapid spontaneous remission.