In the majority of mesothelioma cases worldwide, asbestos is a likely causal factor, but several alternative factors, such as ionizing radiation, have been recognized. We reviewed ionizing-radiation evidence from epidemiology studies of (1) patients exposed to the diagnostic X-ray contrast medium "Thorotrast," (2) patients undergoing radiation therapy (i.e., to treat cancer), and (3) atomic energy workers chronically exposed to lower levels of radiation. The results from these populations are also supported by case reports of mesothelioma following therapeutic radiation. Statistically significant associations were found in many, but not all, epidemiology studies (particularly those of Thorotrast- and radiation-treated patients). Given the low mesothelioma rate in the general population, the consistently increased risk among these radiation-exposed individuals is noteworthy. Many studies were limited by the lack of a uniform manner in which mesothelioma was reported prior to introduction of a uniform classification system (ICD-10). Future studies that rely on ICD-10 should have greater power to detect an association. While the evidence falls short of a definitive causal link, considering studies in which statistical significance was achieved, the case reports, and the plausible mode of action, we conclude that the evidence is supportive of a causal link between ionizing radiation exposure and mesothelioma risk.