The development of autoantibodies during interferon therapy is frequent, but clinical symptoms of autoimmune disease are uncommon. We report on a female patient who developed arthritis with strongly positive antinuclear factor (ANA) and autoimmune thyroiditis while being treated with alpha 2b interferon (IFN) for chronic myelocytic leukaemia (CML). The arthritis subsided promptly after discontinuation of IFN and initiation of nonsteroidal anti-inflammatory drugs.