A 62-year-old man developed resistance to coumarin congeners during treatment with phenprocoumon (Marcoumar) and acenocoumarol (Sintrom), requiring at least triple the usual therapeutic dose and plasma concentration of phenprocoumon for satisfactory anticoagulation. After 8 months on this high dosage, the patient suddenly developed haemorrhagic diathesis. The medication was therefore discontinued. The resistance could not be ascribed to impaired absorption or enhanced metabolism.