Besides hip dysplasia diagnosed after birth, there are dysplasias that do not develop until puberty, causing subluxation of the femoral head to occur late in the skeletal growth period. These dysplasias have various causes. In the two siblings described in this study, the boy showed a conspicuous delay in the appearance of the femoral nuclei and the fusion of the triradiate cartilages. The fusion occurred at 16 years of age in the boy and at 13–14 years in the girl. This was preceded by conspicuous structural changes, especially in the posterior triradiate cartilage and acetabular roof but also affecting the lateral growth plate of the femoral neck, which was horizontal over two-thirds of the diameter. We know from animal studies that the growth of the triradiate cartilage increases the diameter of the acetabulum but not the depth. The acetabulum is deepened by pressure from the femoral head. As a result of coxa valga and a prolonged period of acetabular expansion, combined with abnormalities of the superior acetabular rim, the femoral heads in these children finally subluxate. Whenever development of the femoral ossific nucleus is delayed during the first year of life, radiographic follow-ups should be instituted at 8 years of age.