A 73-years-old man, who was a farmer, developed fever, fatigue, and muscle weakness. His serum creatine kinase (CK) level was found to be high. Computed tomography showed the presence of bibasilar subpleural interstitial infiltrates. He was therefore clinically given a diagnosis of polymyositis. After high-dose glucocorticoid (GC) and GC-pulse therapies were started, his symptoms improved, and the CK level decreased. The patient's son was 44-years-old. He moved back into his parent's home, quit his corporate job and started working in the agricultural field. He then developed fever, myalgia, and muscle weakness. His serum CK level was high. Therefore, he also was given a diagnosis of polymyositis. After high-dose GC and immunosuppressive therapies were stared, his symptoms improved and CK level decreased. Polymyositis may be triggered by specific environmental exposures in genetically susceptible individuals. The son developed the disease immediately after he moved into his parent's home and changed his profession to farming which was same as his father's profession. Moreover, the father and his son may have had common genetic factors. We believe that some environmental factors triggered the onset of polymyositis in the father and his son.