Anti-Mi-2 antibody-positive lung cancer-associated polymyositis.

After a 75-year-old man was diagnosed with lung cancer, proximal weakness and myalgia in the bilateral lower extremities developed, and the creatinine kinase (CK) level was elevated. The anti-Mi-2 antibody test was positive, muscle T2-weighted/fat-suppressed magnetic resonance imaging showed high intensity, and there were no skin lesions. Therefore, he was diagnosed with lung cancer-associated polymyositis (PM). The lung tumour shrank after chemotherapy, accompanied by gradual improvement of his PM-derived symptoms and CK level. Although positive anti-Mi-2 antibody tests rarely indicate PM and cancer, examining myositis-specific autoantibodies, including anti-Mi-2, should be considered if the CK level increases after a cancer diagnosis.