1. A case of tendinitis ossificans traumatica is reported. We believe that it was definitely due to a proliferation of the periosteum resulting from trauma. This is a rare condition; only eight cases of similar nature have been reported. So far as we are able to learn the patellar tendon has not been the site of such a process in any of the reported cases.
2. Two cases of a rarer type, apparently due to an actual metamorphosis, have been reported.
3. In the proliferative type, surgical treatment to remove the mass, replace a functional mechanism, and prevent further proliferation is necessary. In the type due to a metamorphosis of tissue, simple excision only is indicated.