A study has been conducted to determine the mortality experience from 1950-1977 of a cohort of women treated for tuberculosis in Canadian sanatoria between 1930 and 1952. Approximately 50 percent of these women received substantial breast tissue doses of fluoroscopic irradiation in conjunction with their treatment by artificial pneumothorax. A preliminary analysis of 23572 women known alive at the beginning of 1950 has shown a highly significant breast cancer mortality risk for those women exposed to such radiation. There is evidence of decreasing effect with increasing age at first exposure, and no increase in risk is observed until ten years after first exposure.