SUMMARY OF BACKGROUND DATA. Hereditary and mechanical factors are considered to be the principal etiologic factors of Scheuermann's disease. OBJECTIVES. The authors report identical twins presenting similar lesions of localized lumbar osteochondrosis, which were worse in the one twin that practiced strenuous sports activities. CONCLUSIONS. These observations suggest, at least in some of these patients, a basic role for genetic factors in the occurrence of the disease and an influence of mechanical strain on its severity.