This clinical lesson presents three patients with an atraumatic swelling of the sternoclavicular joint. Two of them, a woman aged 76 and a man aged 75, had osteoarthritis of the sternoclavicular joint, and the third patient, a 52-year-old woman, had ossal metastasis from a follicular thyroid cancer. Osteoarthritis of the sternoclavicular joint occurs rather frequently and degenerative changes may eventually result in spontaneous dislocation of the affected joint. However, the differential diagnosis of atraumatic swellings of the sternoclavicular joint can be troublesome and includes among other things: synovitis-acne-pustulosis-hyperostosis-osteitis (SAPHO) syndrome, sternocostoclavicular hyperostosis, osteitis condensans, aseptic necrosis, Tietze’s syndrome, spontaneous subluxation, septic arthritis, gout arthritis, rheumatic arthritis, and primary or metastatic malignancies. In typical presentations of sternoclavicular osteoarthritis, patients can be reassured without additional diagnostic investigation. If there is doubt, CT and MRI scans are helpful in establishing the diagnosis.