The authors describe coxa plana in the dog with the aid of clinical examination, roentgenograms, histological examination, and microangiographs. In both dog and man the condition appears basically to be an aseptic necrosis in the capital femoral epiphysis with all its typical histological characteristics. Microangiography reveals the non-vascular state of the epiphysis together with hypervascularization of all the surrounding tissue. The hypervascularization probably represents an attempt to revascularize the dead bone, but this is delayed by the capital femoral epiphysis being poorly accessible to blood vessels because of the articular cartilage and the avascular epiphyseal cartilage.