From 1990 through 1993 nine patients presented with unexplained swelling of the sternoclavicular joint. In some cases a tumor was suspected. The clinical histories included trauma with injury of forearm and wrist or strain on the shoulder girdle without acute onset of any symptoms in the sternoclavicular joint. In trauma cases the time from injury to onset of symptoms was between 1 day and 1 year (median 5 weeks). CT studies demonstrated widening of the joint space in two cases, thickening of the pectoral muscle in six cases and thickening of capsule and ligaments in five cases. In one case arthroplasty of the sternoclavicular joint was performed and in a second we performed a biopsy. The remaining cases were not treated. Other differential diagnoses to be considered in the case of a swelling of the sternoclavicular joint are listed.