History and clinical findings: A 74-year-old man known to be suffering from a chronic hepatitis C infection was hospitalized because of intestinal hemorrhage, multiple petechiae and suggillations. The patient haul received oral anticoagulant medication after replacement of the mitral valve. The intake of oral vitamin It antagonist had been discontinued eight days before admission. Examination findings: On admission, the platelet count was 3 G/l, the Quick's test 72%. Colonoscopy revealed diffuse mucosal bleeding in the proximal colon. Bone marrow examination showed moderate hyperplasia of erythropoiesis, as well as a marked increase in megakaryocytes. Diagnosis, therapy and course: The diagnosis was hepatitis C-associated idiopathic thrombocytopenic purpura. The administration of both immunoglobulins and prednisone failed to increase platelets sufficiently. There was also no improvement after administration of Danazol. However, six days after a single application of cyclophosphamide (2,000 mg on day 1), a continuous increase of platelets to 100 G/l was obtained, a level which has remained stable for the last 18 months. Conclusion: The differential diagnosis of a hepatitis C-associated autoimmune thrombocytopenia must be considered in patients with chronic hepatitis C infection and severe thrombocytopenia. In cases of refractory disease, treatment with cyclophosphamide may be successful. This is particularly appropriate if, in the case of an insufficient increase in thrombocytes following administration of immunoglobulins, the success of a splenectomy is improbable, or when a splenectomy must be excluded because of relative or absolute contraindications.