1. A rare cancerous tumor of the smooth muscle cells. It is most often found in the uterus or abdomen.
2. A sarcoma (malignant tumor that begins growing in connective tissue such as muscle, bone, fat, or cartilage) containing large spindle cells of smooth muscle.

Although it rarely occurs in soft tissue, it is common in the viscera. It is the most common soft tissue sarcoma of the gastrointestinal tract and uterus. The median age of patients is 60 years.