Five patients had persistent, pulling pain in the sternum, clavicles, and upper ribs that was exacerbated by cold and dampness. Clublike, symmetrical enlargement of the clavicles was seen and, in two patients, venous congestion of the upper half of the body. All patients had a constantly elevated erythrocyte sedimentation rate. Radiologically there was symmetrical hyperostosis of the sternal and middle portions of the clavicles, synostosis of the sternoclavicular joints, a widened and thickened sternum, and varying degrees of involvement of the upper ribs. X-ray findings did not change over several years. Phlebography showed bilateral subclavian vein occlusion in three patients and unilateral occlusion in one. Biopsies of the clavicles showed a characteristic hyperostotic sclerosis of the spongiosa. The cause of this sternocostoclavicular hyperostosis is unknown, but clinical, radiologic, and histologic findings indicate that it may represent a distinct, hitherto undescribed entity.