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We describe a 32-year-old man with secondary antiphospholipid syndrome (APS) who received oral anticoagulation with phenprocoumon after deep vein thrombosis. Conventional monitoring of oral anticoagulation by INR measurement was impaired by coagulation factor inhibition in vitro due to a strong lupus anticoagulant. The case illustrates that monitoring of oral anticoagulation may require determination of single coagulation factor activities in selected patients with APS.