A 68-year-old man visited his home doctor with the chief complaint of left flank pain. Abdominal ultrasonography revealed left hydronephrosis and space occupying lesion in the right kidney. He was referred to our hospital because of suspected left ureteral stone and for further examination. Drip infusion pyelography revealed a left ureteral stone and retrograde pyelography showed extrinsic compression and deformation of the right pyelogram. A computed tomography showed a heterogeneous solid mass, 7 x 4 cm in size, compressing the pelvicocalyceal system and growing out of the kidney. Right radical nephrectomy was performed under the diagnosis of renal cell carcinoma. The pathological diagnosis was leiomyosarcoma. Seventy-five cases of renal leiomyosarcoma have been reported in the Japanese literature and this is the third case as an incidental tumor. Because of the absence of the characteristic diagnostic signs, this malignant tumor can present diagnostic difficulty. The diagnostic problems and clinical features of renal leiomyosarcoma are discussed.