The reliability of exposure data directly affects the reliability of the risk estimates derived from epidemiological studies. Measurement uncertainty must be known and understood before it can be corrected. The literature on occupational exposure to radon (222Rn) and its decay products reveals only a few epidemiological studies in which uncertainty has been accounted for explicitly. This work examined the sources, nature, distribution and magnitude of uncertainty of the exposure of French uranium miners to radon (222Rn) and its decay products. We estimated the total size of uncertainty for this exposure with the root sum square (RSS) method, which may be an alternative when repeated measures are not available. As a result, we identified six main sources of uncertainty. The total size of the uncertainty decreased from about 47% in the period 1956–1974 to 10% after 1982, illustrating the improvement in the radiological monitoring system over time.