Even in the pioneer days of radiation therapy the problem of dental caries as a sequel to irradiation of lesions in the oral cavity existed. Despite technical advances and increasing experience, the problem persists. In many centers pretreatment extraction of teeth has become an established procedure to avoid subsequent dental complications. Yet disastrous mandibular necrosis as a complication to extractions after irradiation was reported as late as 1952 (1). Although bone has been considered relatively radioresistant, when it is in the region of a cancer it may be the limiting factor in the amount of radiation that can be delivered. In the oral cavity, a tooth-bearing bone in the treatment field provides an excellent mechanism for the introduction of infection and subsequent osteonecrosis if and when caries appears. After the vascularity of the bone has been compromised by irradiation, little response to bacterial invasion occurs, and the necrosis slowly progresses to involve the entire irradiated bony part, in patients who are otherwise free of disease. Case material from the Tumor Institute of the Swedish Hospital reported in this study will be analyzed to assess the value of pre-irradiation extraction of teeth, as a method of preventing not only post-irradiation caries and pain, but also the more disastrous complication of osteonecrosis of the mandible. A method of avoiding mandibular necrosis is suggested by this study, and its rationale will be developed. The Problem of Osteonecrosis Regaud reported on the complication of mandibular necrosis following irradiation as early as 1922 (2). He observed that it differed from typical osteomyelitis in that there was no line of demarcation between the dead and viable bone. When the unrelenting process had reached the boundary of the field of irradiation, the sequence of involucrum formation, sequestration, and healing proceeded in the ordinary fashion. He postulated that the loss of reactivity of bone compromised by irradiation was due to the decreased vascular reserve from endothelial proliferation. These observations are frequently substantiated today. A review of 1,819 records of patients irradiated for cancer of the oral cavity was published by Watson and Scarborough of the Memorial Hospital (New York) in 1938 (3). They were unable, however, to state the percentage of these patients completing treatment, the amount of irradiation received by those who did, or their pre-treatment dental status. Consequently, the frequency figure for osteonecrosis based on this series—12.9 per cent of all patients accepted for irradiation therapy of intra-oral cancer—is of little aid in determining the responsible factors. Irradiated Bone In 1950, McCrorie (4) postulated that the post-irradiation osteoporosis observed on roentgenographic studies resulted from unopposed osteoclastic activity, since microscopically he saw no surviving osteoblasts.