Although there is good evidence that D-penicillamine can induce polymyositis, the exact pathogenic mechanism remains unclear. We report two patients with psoriatic arthritis and primary biliary cirrhosis respectively, who developed polymyositis while receiving D-penicillamine treatment for their primary diseases. Whether D-penicillamine treatment was the sole cause of polymyositis or acted as a trigger for the development of a secondary autoimmune disease is discussed.