Dermatomyositis is a clinical syndrome of unknown etiology characterized by a nonpurulent inflammatory myopathy involving striated skeletal muscle associated with rheumatoid arthritis, or systemic lupus erythematosus, sometimes underlying malignancy. A few cases of acute renal failure due to rhabdomyolysis in dermatomyositis has been reported. But, no case combined with breast cancer have been reported. We experienced a case of acute renal failure due to rhabdomyolysis associated with dermatomyositis. The patient was admitted to the our hospital due to breast cancer. After breast cancer operation, she complained of progressive muscle weakness. The heliotrope rash involved the eyelid, bridge of nose and forehead. And eczematoid dermatitis involved the lower abdomen and both forearm and hands. The laboratory findings revealed LDH 2,944IU/L, CPK 2,244IU/L and SGOT 214IU/L. Serum myoglobin and aldolase were increased, and antinuclear antibody was 1:40(positive, homogenous pattern). Electromyogram revealed myopathic findings compatible with dermatomyositis. Then, the patient developed acute renal failure requring hemodialysis. After acute hemodialysis, acute renal failure resolved. But, the patient died on the 113th hospital day due to sepsis. Here, we report a case of acute renal failure due to nontraumatic rhabdomyolysis associated with dermatomyositis in breast cancer.