Federal Guidance Report No.13 (FGR 13) provides cancer risk coefficients for various modes of environmental exposure to each of more than 800 radionuclides (EPA 1999), including inhalation of airborne activity and ingestion of activity in food or drinking water. A risk coefficient for inhalation or ingestion of a radionuclide is an estimate of the probability of radiogenic cancer mortality or morbidity per unit activity taken into the body. A risk coefficient may be interpreted either as the average risk per unit intake for persons exposed throughout life to a constant activity concentration of a radionuclide, or as the average risk per unit intake for persons exposed briefly to the radionuclide in an environmental medium. The risk coefficients in FGR 13 apply to an average member of the public in the sense that estimates of risk are averaged over the age and gender distributions of a hypothetical population whose survival functions and cancer mortality rates are based on recent data for the U.S. The present report updates the uncertainty analysis in FGR 13 for the cases of inhalation and ingestion of radionuclides and expands the analysis to all radionuclides addressed in that report. This analysis is in response to suggestions that the uncertainty analysis in FGR 13 should be expanded as an aid in determining the kinds of additional information that are most needed for improving confidence in risk assessments for exposure to environmental radionuclides.