We have developed a procedure to transplant free autogenous iliac-crest physeal grafts into defects created in the lateral aspect of the distal femoral physis of rabbits. Excision of a portion of the lateral part of the physis led consistently to formation of a bone bridge, growth arrest, and valgus deformity. There was no evidence of increased activity by the persisting physis to fill the defect or of differentiation of mesenchymal cells into a functioning physeal layer. We carried out preliminary studies to define the optimum conditions for obtaining and transplanting a free autogenous iliac-crest physeal graft into the femoral physeal defect. The iliac apophysis is composed of a fibrocartilaginous layer, cartilage similar to epiphyseal cartilage, and a physis. The graft included the physis and some overlying epiphyseal cartilage but the outermost fibrocartilaginous layer and the surrounding perichondrium were removed, as these two tissues were believed to inhibit diffusion. The graft was separated gently from the metaphysis. Histological studies showed that the separation occurred at the lower hypertrophic-chondrocyte zone. After optimum techniques had been developed for transplantation and the feasibility of the procedure had been established in twenty-one rabbits, the capacity of the physeal transplants to prevent bone-bridge formation, growth arrest, and valgus deformity was evaluated in thirty-nine rabbits, three to four months old, using the standardized transplantation procedure. The results were assessed in terms of the gross and radiographic appearance of the femur, measurements of length, and histological characteristics. The transplanted physis retained its normal morphology, united with the residual part of the original femoral physis, and participated in endochondral bone formation. Bone-bridge formation between the femoral epiphysis and metaphysis, growth arrest, and valgus deformity were prevented or minimized in most animals, and the physeal transplants yielded good or excellent results in 60 per cent of the procedures using stringent criteria. The results of this study show that free physeal transplants into focal defects can remain viable and function satisfactorily if they are fashioned properly and inserted gently. CLINICAL RELEVANCE: This study demonstrated that transplantation of an iliac-crest physis into a defect of the lateral aspect of the distal femoral physis can prevent bone-bridge formation, growth arrest, and the development of valgus deformity.(ABSTRACT TRUNCATED AT 400 WORDS)