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Primary osteosarcoma of the kidney is very rare. Its exact histogenesis remains unclear. It has a tendency to recur locally and metastasize, and the prognosis is very poor. We present a case of a 59-year-old woman with left flank pain and weight loss. Abdominal ultrasonography and bone scan revealed a large solid retroperi-toneal mass with calcifications. The patient underwent radical nephrectomy; microscopic examination showed atypical cells with the characteristic pattern of classic osteosarcoma with immature neoplastic osteoid. Ten months later, the patient developed metastatic lesions in the liver, without local recurrence or other sites of metastases. The patient is currently receiving chemotherapy with doxorubicin and cisplatin, with a follow-up in 17 months. The most important clinical findings, the pathogenesis and the treatment modalities of this rare neoplasm are discussed.