OBJECTIVE To evaluate the risk of breast cancer among women occupationally exposed to ionizing radiation. METHODS Case-control study. METHODS A health survey of 105,385 women radiologic technologists certified by the American Registry of Radiologic Technologists since 1926. Among 79,016 respondents, 600 breast cancer cases were identified. Each of 528 eligible subjects with breast cancer was matched to five control subjects based on age, year of certification, and follow-up time. METHODS Relative risk (RR) estimated as the relative odds ratio for breast cancer over categories of years worked as a radiologic technologist and according to personal and occupational exposure characteristics. RESULTS Study subjects had been certified for a mean of 29 years; 63.8% of cases and 62.6% of controls worked as radiologic technologists for 10 years or more. Significant increased risks for breast cancer were associated with early age at menarche (for < 11 years of age: RR = 1.79; 95% confidence interval [CI], 1.09 to 2.94), nulliparity (RR = 1.36; 95% CI, 1.04 to 1.78), first-degree relative with history of breast cancer (RR = 2.07; 95% CI, 1.56 to 2.74), prior breast biopsy (RR = 1.53; 95% CI, 1.17 to 2.00), alcohol consumption (for > 14 alcoholic drinks per week: RR = 2.12; 95% CI, 1.06 to 4.27), thyroid cancer (RR = 5.36; 95% CI, 1.64 to 17.5), hyperthyroidism (RR = 1.66; 95% CI, 1.02 to 2.71), and residence in the northeastern United States (RR = 1.66; 95% CI, 1.19 to 2.30). Jobs involving radiotherapy, radioisotopes, or fluoroscopic equipment, however, were not linked to breast cancer risk, nor were personal exposures to fluoroscopy or multifilm procedures. Use of birth control pills, postmenopausal estrogens, or permanent hair dyes also were not risk factors. Based on dosimetry records for 35% of study subjects, cumulative exposures appeared low. Among women who worked more than 20 years, the RR for breast cancer was 1.13 (95% CI, 0.79 to 1.64). CONCLUSIONS More than 50% of the reported breast cancers could be explained by established risk factors. Employment as a radiologic technologist, however, was not found to increase the risk of breast cancer. The contribution of prolonged exposure to relatively low doses of ionizing radiation to breast cancer risk was too small to be detectable at this time.