1. Minimal trauma to the shaft or to the metaphysis of a long bone, with or without interruption of the medullary blood supply, does not produce any definite increase in rate or extent of longitudinal bone growth.

2. Gross trauma, involving a considerable portion of the shaft of a long bone and necessitating extensive repair over a long period of time, does produce epiphyseal stimulation and longitudinal overgrowth.

3. The increased rate of growth continues only during the period of healing.

4. This growth stimulation appears to be secondary to the hyperaemia and increased vascularity of the bone and soft parts following the trauma and during the process of healing.

5. The longitudinal overgrowth which may accompany a fracture occurs regardless of whether or not there is initial shortening as a result of overriding or loss of shaft substance. This observation, together with the fact that overgrowth may also occur after gross trauma without loss of continuity of the shaft of a bone, such as excision of a full-thickness tibial bone graft, is further evidence to support the theory of local hyperaemia as the basic stimulus and that it is not a compensatory phenomenon.

6. Evidence that longitudinal bone growth may occur from the diaphysis or from the cartilage of the callus following fracture of the shaft is not well supported by the data presented.

7. Chronic localized inflammation of the diaphysis, due to an irritant foreign body, may result in marked bone proliferation with increased diameter of the shaft. There is some evidence that under these circumstances localized interstitial longitudinal growth may occur. This possibility is deserving of further investigation.