A 29-year-old male patient with chronic hepatitis C infection and interferon alpha therapy in his medical history was admitted to the hospital because of the clinical manifestation of a pulmonary renal syndrome. High titers of proteinase-3-ANCA were detected, while an infectious agent was ruled out. After diagnosis of Wegener's granulomatosis the patient received prednisolone and cyclophosphamide pulse therapy and remission developed rapidly. Chronic hepatitis C infection as well as interferon therapy are frequently associated with autoimmune disorders. We assume that the interferon therapy itself has triggered autoimmune processes resulting in Wegener's granulomatosis in our patient. Thus we recommend to search specifically for autoimmune disorders in the past medical history and if necessary to consider a screening for autoantibodies before starting an interferon therapy. An autoimmune disease should also be taken into account if new symptoms develop under an ongoing interferon alpha therapy.