The familial tendency of Legg-Perthes disease has been well documented by the family incidence and even more strongly by an extensive study of family pedigrees. The twins studied by Giannestras and by Dunn and this report of the disease which is present in mirror fashion in identical twins illustrate even more vividly the genetic interaction in the disease. Therefore, those who are aware that coxa plana has appeared in a family should keep in mind the probability of the disease developing in another member.