Ossification of the Achilles tendon is a rare condition to be distinguished from the more frequently occuring tendon calcification. It is characterized by the presence of an ossific mass contained within the fibrocartilaginous substance of the tendon and is usually associated with prior surgery or trauma to the tendon. We treated a 13 year old girl with ossification of the Achilles tendon after sprain-like injury. We excised it one year later and the microscopic finding of the extirpated bony mass revealed fully maturated lamellar bone.