INTRODUCTION: Radiotherapy is useful in the treatment of breast cancer of all stages. Cobalt radiotherapy has been used since the 1950s for breast-cancer management. However, since the advent of linear accelerators, its use has diminished. Post mastectomy radiotherapy has proven survival benefits especially in patients with >4 positive lymph nodes. Among node-positive patients there was a 37% relative reduction of distant metastases with radiation and a 22% relative reduction of death(1). The occurrence of short term and long term complications from exposure to the cobalt radiotherapy including recurrence of cancer, arm paralysis, recurrent laryngeal nerve paralysis and neuropathy depend largely upon the duration of exposure, volume of organs involved and the exposure site(2). We present an extremely rare case of osteomyelitis post cobalt radiotherapy. To our knowledge, this is the first case to be reported of osteoradionecrosis and osteomyelitis developing as a chronic complication post radical mastectomy and cobalt radiotherapy.