A 21-year-old female American Saddle Horse with a prominent swelling on the ventral aspect of the right hemimandible and radiographic evidence of a bone cyst was examined. The cyst cavity was surgically explored and was found to contain mucinous fluid and sheets of keratin. Microscopically, the cyst was lined by well-differentiated squamous epithelium characterized by various degrees of hyperkeratosis and parakeratosis. The stromal component varied from scant areas in which epithelium was closely apposed to bone to wide regions of dense connective tissue that contained fragments of keratin surrounded by foreign-body giant cells. Histologic features were consistent with a diagnosis of intraosseous epidermoid cyst. After curettage of the cyst and repulsion of the second premolar tooth, drainage was established and iodine flushes were begun. Nine months after surgery, drainage was not detectable and size of the hemimandible had decreased by approximately a fourth.