Leiomyosarcoma of the left kidney seen in a 58-year-old man is reported. On April 10, 1982, he complained of left flank pain. He visited our hospital and left solitary renal cyst was suspected. He had been treated as an outpatient, but left flank pain became exacervated. On May 18, he was admitted to our hospital. On June 7, radical nephrectomy was done under the diagnosis of left renal cell carcinoma. At operation, the tumor invased directory to the psoas muscle and abdominal wall, and could not be completely resected. Pathological diagnosis was renal cell carcinoma with sarcomatoid change. On July 1, he was discharged from the hospital. In December, left flank distention appeared and back pain became exacervated. On February 8, 1983, he was readmitted to our hospital. Low density area was found in left psoas muscle by CT scanning and recurrence of renal cell carcinoma was suspected. alpha-Interferon therapy had been done, but tumor increased remarkably and caused ileus. He died on June 14, 1983. The autopsy revealed a child head-sized cystic tumor in the upper retroperitoneal space, a 5 X 5 X 5 cm metastasis of the left lobe of the liver, a 3 X 3 X 4 cm tumor to the left upper lobe with cavity formation and direct invasion into the spleen, diaphragma and gastric serosa. These metastatic lesions were leiomyosarcoma. Retrospectively, the primary tumor of kidney revealed primary leiomyosarcoma of kidney.