[en] Cytogenetic and moleculobiological analyses were made for the relationships between leukemogenesis and both A-bombing and myelodysplastic syndrome (MDS) in two A-bomb survivors developing MDS. Both of the survivors had entered the city early after A-bombing (two days and seven days later). They had a normal karyotype of bone marrow cells, but had point mutation of the N-ras proto-oncogene. In one survivor, MDS (RAEB-t) was transferred to leukemia; and in the other survivor, leukemia was not observed during a follow-up of more than one year. There was no definitive correlation between A-bombing exposure and the development of MDS in either survivor. Point mutation of the N-ras proto-oncogene may be a possible factor, but not a critical factor, for developing leukemia. (N.K.)