1. Thirty-two patients with segments of bone taken from the ipsilateral fibula as grafts for the Batchelor type of subtalar fusion were analyzed for ankle instability in relation to regeneration of the donor site and elevation of the lateral malleolus.
2. It was found that, in the growing child, failure of regeneration of the fibula resulted in progressive elevation of the lateral malleolus and led to valgus instability of the ankle.
3. It was also found that the lower third of the fibula was less prone to regeneration than the middle third. An attempt was made to explain why this was so on the basis of blood supply.
4. Older children and adults showed a lesser potential for fibula regeneration than did younger subjects.