We are reporting here a case of chondrosarcoma of the soft tissues of the thoracic wall. As far as we can tell it is the first case published in the world. The tumour appeared 13 years after radical mastectomy and radiotherapy for an adenocarcinoma of the breast. The tumor was greyish-white in appearance and hard, but friable and haemorrhagic. Clinically and biologically it looked like an inflammatory mass. This disappeared when the tumour had been removed but recurred 6 days post-operatively when nodules showing that it had permeated the tissues appeared. Histological and immunohistochemical examination confirmed the diagnosis. We discuss the clinical and histological aspects of the case and state what we found in the literature about other sarcomas that have followed adenocarcinomata of the breast that had been irradiated and operated on. The role of radiotherapy in bringing about this tumour is discussed, including the relationship between the site of the tumour and the dose and field of irradiation. The prognosis is particularly bad in this type of sarcoma which was induced by irradiation.