The qualitative leukemogenicity of ionizing radiation was firmly established by studies of medical workers and patients exposed to high radiation levels in the mid-1900s. Quantitative relationships were evaluated through extensive studies of atomic bomb survivors and patients who received therapeutic radiation, for whom the duration of exposure was brief. Although many studies have been conducted of nuclear workers and others exposed occupationally, uncertainty remains about quantitative aspects of the leukemia-radiation exposure relation for low dose-rate, fractionated exposures. Some studies have shown dose-related increases in leukemia risks for certain nuclear workers in the U.S. and Europe, although these findings are inconsistent across populations. Despite limitations in low-dose epidemiology, well-designed studies among nuclear workers should inform some controversial aspects of the relation between ionizing radiation exposure and leukemia risk.