肉瘤只組成所有腎臟腫瘤的2-3%，其中以平滑肌肉瘤是最常見的一種，平滑肌肉瘤大部份以腰部腫瘤，腰痛，血尿，體重減輕，或腸胃不適來表示。我們報告一例罕見的平滑肌肉瘤，以自發性腎周圍血腫呈現。

Sarcoma constitutes only about 2 to 3 percent of malignant tumors of the kidney. Leiomyosarcoma is its most common variety, and mostly presents with flank mass, flank pain, hematuria, weight loss or GI disturbance. A rare case of primary renal leiomyosarcoma presenting with spontaneous perirenal hematoma was reported.