A large (50 x 25 x 20 cm), complex echoic, ventral abdominal mass was detected by ultrasound examination of an 18 year old thoroughbred mare with a history of chronic ill-thrift and colic. Histologically, the mass consisted of monomorphic streaming spindle cells in a slightly myxomatous stroma. The majority of tumor cells demonstrated diffuse cytoplasmic positive immunoreaction for vimentin and smooth muscle actin and negative reaction for c-kit (CD117). Based on histology and immunohistochemistry, the mass was determined to be a myofibroblastic tumor or a leiomyoma. The mare was managed medically and survived for 3 months until an episode of severe colic necessitated euthanasia on the farm. No necropsy was performed.