Leidocranial dysostosis is a condition wherein single or multiple abnormalities of osseous development occur spontaneously or from inheritance. It is so named because all the patients in whom it has been found have had defects of the clavicles and most of them have had skull deformities. An excellent review of the condition up to 1929 was given by Fitchet (1). Other references are listed in the bibliography. The following abnormal conditions have been reported: Skull and Head.—In cases with skull and bone defects, radiographs of the skull may show large areas not ossified, failure of fusion at the fontanelles and sutures, and the presence of multiple wormian bones. The patency of the fontanelles may be permanent. Metopic sutures are common. Failure of ossification along the sagittal suture may occur. There may be openings at the bregma and asterion. One defect of the right orbital ridge has been reported. The occipital foramen may be tilted forward and be higher than normal because of deformed sphenoidal and facial bones. The bones of the base of the skull and face are small with small nasal accessory sinuses. The jaws are often prognathous. There may be an absence of the nasal and lachrymal bones. The deciduous teeth persist longer than normal—up to 12 years—and there is delayed eruption of the permanent teeth which may have faulty implantation and enamel defects. There may be an absence of some of the permanent teeth, particularly the canines, or development of supernumerary teeth. Shoulder Girdle.—Defective formation of one or both clavicles is the most common finding from roentgen examinations. In a few cases the clavicular defects are the only abnormalities found. A variety of changes in the clavicles, from a simple transverse defect of the middle third, sometimes mistaken for an ununited fracture, to complete absence of both clavicles, have been reported. In the majority of cases the defects are found at the sternal ends, but examples in which the acromial ends have been missing have been illustrated. A thin fibrous section, from a failure of union of the medial and outer halves, may exist in the shaft. In these cases the opposing ends are rounded and generally overlap. The acromial processes may show abnormalities with absence of the articular surfaces at the acromioclavicular joints. Absence of the supraspinous fossa of the scapula may occur, in which case the root of the spine forms the upper border of the scapula. Arms and Forearms.—Partial or complete absence of the radius has been reported. The styloid processes of the radius and the ulna may be at the same level, whereas normally the radius is longer. Hands.—Roentgenograms of the hands show multiple deformities which are characteristic of the condition. The distal phalanges are without their ungual expansions; they are short and conical in shape. The middle phalanges are much shorter than normal and their lateral borders are concave, almost resulting in a dumb-bell shape. The proximal phalanges are expanded in their proximal halves, and there is some narrowing of the shafts with increased density of the middle and distal thirds. The metacarpal bones are expanded at their extremities and narrowed in the middle thirds, with some increase in the compact tissue at this level. Epiphyses frequently are seen at both ends of the basal phalanges and metacarpal bones. One case of elongation of the second metacarpal bone has been reported. Spine.—Several abnormalities in the development of the spine have been reported. Kyphosis, lordosis, and scoliosis may occur. Spina bifida has existed. Cervical ribs are common. One patient with an extreme scoliosis showed bifed bodies of the seventh to eleventh thoracic vertebrae and defective laminae in the fourth and fifth lumbar and all the sacral segments. Pelvis.—The pelvic bones frequently are abnormal in development. Before puberty the pelvic canal is narrowed and the joint spaces are considerably widened, including the spaces between the ischium, ilium, and pubis at each acetabulum. After puberty the pelvic joint spaces are narrowed and irregular in outline, the mature pelvis having the broad, squat appearance associated with malformed, defective joints. The pubic bone may be missing. In two sisters with cleidocranial dysostosis, there were defective laminae of the first sacral segment.