Computed tomographical analysis of sternocostoclavicular hyperostosis was performed in 27 patients. In the earliest stage hyperostosis occurred around the cartilaginous portion of the first ribs. The sternoclavicular joint space was preserved even in the late stage III of the disorder. These findings suggest that sternocostoclavicular hyperostosis develops around the costal cartilage including periosteum, perichondrium, and the ligamentous structures, and that the sternoclavicular joint is not primarily involved. It is also suggested that perichondritis and periostitis play important roles in the etiology of this disorder.