The term “windswept deformity” describes the appearance of abnormal valgus deformity of one knee in association with varus deformity of the other. It is commonly seen in young children in certain part of Africa. But the deformity occurred in adult is rare. We retrospectively reviewed five adult patients who had windswept deformity to investigate the reason of occurrence of this deformity. All five had neither abnormality of conditions of the hip joints nor difference in length between each lower extremity. One had right convex lumbar scoliosis. Torn discoid were found in two cases which appeared to be, responsible for valgus angulation of the knee. Knee pain of valgus side occurred a few years earlier than that of the varus side in four cases, indicating that a valgus deformity developed on the “windward” side at first followed by the appearance of the varus deformity in the other “leeward” knee. It is suggested that one side valgus angulation might cause a compensatory varus angulation on the contra-lateral side.