"This report is intended to draw attention to the points of misunderstanding and to clarify them in the light of the information gained from a thorough study [of transchondral fractures of the talus].... A transchondral fracture is a fracture of the articular surface of a bone, produced by a force transmitted from the articular surface of a contiguous bone across the joint and through the articular cartilage to the subchondral trabeculae of the fractured bone. Either of two physical types of fracture may result. First, the fracture may result in a small area of compressed trabeculae, with or without demonstrable damage to the overlying cartilage. Or it may be an avulsion of an osteocartilaginous flake." Anatomic studies on cadaver limbs demonstrated the etiological mechanism of transchondral fractures of the lateral border of the talar dome. As the foot is inverted on the leg, the lateral border of the dome is compressed against the face of the fibula (Stage One), while the collateral ligament remains intact. Further inversion ruptures the lateral ligament and begins avulsion of the chip (Stage Two), which may be completely detached but remain in place (Stage Three) or be displaced by inversion (Stage Four). We believe that the traumatic etiology of the lesion has been confirmed both clinically and experimentally. The problems of clinical and roentgenographic diagnosis are discussed and illustrated by case histories. Various forms of treatment are evaluated, and the prognosis appraised.