It is well known that leukemia is the first radiation-induced malignancy in atomic bomb (A-bomb) survivors. Recently, two other hematological disorders, myelodysplastic syndrome (MDS) and monoclonal gammopathy of undetermined significance (MGUS), are of great concern among A-bomb survivors because both, the so-called pre-leukemia and pre-myeloma, respectively, are frequently identified in the elderly. However, these diseases have not well investigated in A-bomb survivors. We are now conducting epidemiological studies on MDS and MGUS with over 50,000 A-bomb survivors. We confirmed 162 MDS cases (the crude incidence rate=10.7 per 100,000) by a retrospective study during 1980–2004. The median age at diagnosis was 71 years. MDS incidence rate was higher in men than in women, and an inverse relationship was observed between the incidence of MDS and the distance from the hypocenter. We detected 1081 MGUS cases (prevalence rate: 2.1%; 95% confidence interval: 1.9–2.2%) through an M-protein screening during 1988–2004. The median age at diagnosis was 68 years. MGUS prevalence rate increased by age at diagnosis or age at exposure and was higher in male than in female. A significant inverse relationship between MGUS prevalence rate and exposed distance or a significant positive relationship between MGUS prevalence rate and exposed radiation dose was seen in only a group exposed under 20 years of age in the multivariate analyses. We suggest that A-bomb radiation may affect the development of MDS and MGUS in A-bomb survivors even more than half a century passed after the exposure. Further detailed analyses are necessary to confirm these results.