The description of a roentgenologic finding as a presumptive sign is always a matter of general interest, and while no claim of infallibility is made for our observation, we have decided to report it in the hope that it will be checked and commented upon by other workers. We have observed this finding in several cases of hyperparathyroidism, both of the primary type and of the secondary type associated with chronic renal insufficiency (renal rickets, renal osteodystrophy, or osteonephropathy). In one case the observation of the sign in a routine chest film suggested the diagnosis to one of us (L. N.), and this was later confirmed by a roentgen survey of the skeletal system, biochemical analysis of the blood, and by the postmortem finding of a parathyroid adenoma. The bone findings in primary hyperparathyroidism (osteitis fibrosa cystica generalisata) and renal osteodystrophy (or renal rickets, as it is called in the older literature) have been adequately described elsewhere (3, 6, 13, and 14), and it is not our purpose to review them here except as they appear in our cases. We wish merely to call attention to changes which we have observed in the acromioclavicular articulations in these cases, consisting of cupping of the outer end of the clavicle, with fraying due to irregular absorption of the articular surface. Similar though less striking changes may be present in the articular surface of the acromion process, and there may or may not be widening of the acromioclavicular joint space. Although we have never observed an epiphysis in the outer end of the clavicle, nor have we seen one described, the changes simulate the metaphyseal cupping, fraying, and decalcification seen in vitamin D-deficiency rickets. The reason for the localization of these changes is not apparent, but presumably it is related to the activity of the shoulder and its associated joints. Grant (8) has described the motions about the acromioclavicular joint with movement of the scapula, and Oppenheimer (12) has demonstrated roentgenographically that there is motion in the acromioclavicular joint during all movements of the arm except pronation and supination. The latter writer stresses the importance of the joint function, stating that the mobility of the arm depends significantly on the mobility of the scapula, and that the mobility of the scapula in turn depends on the mobility of the shoulder girdle in the sternoclavicular and acromioclavicular joints. The following cases illustrate the changes described as they appear in the various types of hyperparathyroidism. The first case is one of renal osteodystrophy in association with congenital bladder neck obstruction leading to marked bilateral hydronephrosis and hydroureters, with severe impairment of renal function. Case 1: A white male, aged 16 years, was found to have had a congenital urinary defect at the age of five years, when he was investigated for polyuria and enuresis.