The influence of occupational exposure to ionizing radiation on risk of radiation-related cancers was studied among Norwegian nurses. A cohort of 43 316 nurses who graduated between 1914 and 1984, and were registered by the Norwegian Board of Health's registry of nurses, was followed up from 1953 through 2002 by linkage to the Norwegian Cancer Registry by unique personal identification numbers. Indicators of radiation exposure were developed from data on work history. Internal analyses were performed with Poisson regression, according to time since first potential radiation exposure, duration of exposure, and period of first exposure, using unexposed nurses as reference group. No clear association was found between exposure to ionizing radiation and cancers of the breast, thyroid, ovary, or leukemia, malignant melanoma or other skin cancer. Increased risk of lung cancer was found in the subgroups of nurses first exposed after 1950 (rate ratio=1.47, 95% confidence interval: 0.97-2.23, 26 cases), and in nurses with less than 20 years since first exposure (rate ratio=3.41, 95% confidence interval: 1.67-6.99, 9 cases), but the most likely explanation was confounding by smoking. No firm evidence that nurses potentially exposed to ionizing radiation had increased risk of radiation-related cancer was found.