Leiomyosarcoma is a type of sarcoma which is a neoplasm of smooth muscle. (When benign, it is calle a leiomyoma.) Smooth muscle cells make up the involuntary muscles, which are found in most parts of the body: in uterus, stomach and intestines, walls of all blood vessels, and skin. It is therefore possible for leiomyosarcomas to appear at any site in the body. It is however most commonly found in the stomach, small intestine and retroperitoneum.