OBJECTIVE: Polymyositis (PM) mainly involves proximal limb and trunk muscles. Ocular muscles are not affected, except in rare cases with both PM and myasthenia gravis (MG). Thus, the results of laboratory examinations in such a patient deserve to be reported.METHODS: To analyze the clinical, imaging and pathology datas on a 65-year-old woman patient with PM with complex symptoms, who presented mainly ophthalmoplegia. The patient consented to all examinations and the hospital Ethics Committee approved the study. The laboratory examinations included creatine kinase (CK), ENA, tumor marker, function of thyroid, cranial MRI, and electromyogram (EMG). Biopsy of the left quadriceps femoris was performed, frozen specimens were stained with hematoxylin and eosin, ATPase, NADH tetrazolium reductase, periodic acid Schiff, oil red O, modified Gomory trichrome and MHC-I, to investigate the pathology of muscle fibersRESULTS: Laboratory results showed: CK, 108.32μkat/L; antinuclear antibody: (+); ENA, (-); tumor marker, (-); normal thyroid function, MRI showed no abnormal signals in brain and extraocular muscles. Electromyography of the bilateral deltoid, biceps brachii, musculus quadriceps fexoris, anterior tibialis showed fibrillation potentials, positive potentials and short-duration, small-amplitude polyphasic potentials on voluntary movements with a full interference pattern on mild exertion. Repetitive stimulation did not result in any increment or decrement in these potentials. A muscle biopsy of the left quadriceps femoris showed many small round muscle fibers without peripheral bundle distribution and apparent myofiber degeneration, necrosis and phagocytosis. There were several focal lymphocyte infiltrations. MHC-I immunohistochemical staining was positive in most fibers revealing inflammatory infiltration of normal fibers with MHC-I expression. CONCLUSION: This patient showed increased CK, typical triad of myopathy in EMG, and apparent degeneration and necrosis in biopsy of quadriceps femoris. Therefore, the diagnosis of PM and involvement of extraocular muscles were definite.