Leukemia was the first malignancy linked to radiation exposure in atomic bomb survivors. Clear evidence of the dose-dependent excess risk of three major types of leukemia (acute lymphocytic leukemia, acute myeloid leukemia [AML], and chronic myeloid leukemia) was found, especially in people exposed at young ages. Such leukemia risks were at their highest in the late 1950s, and declined gradually thereafter over the past 50 years. Findings from recent risk analyses, however, suggest the persistence of AML risk even after 1990, and evidence of increased risk of myelodysplastic syndromes (MDS) due to atomic bomb radiation has recently been shown. High-risk MDS and forms involving complex chromosomal aberrations were found to be much more frequent in people exposed to higher radiation doses. These lines of epidemiological evidence suggest that the risk of radiation-induced hematological malignancies has persisted for six decades since the initial exposure.