A chromosome analysis was performed on cultured bone marrow fibroblasts (CFU-F) from two atomic bomb survivors exposed within 1 km of the hypocenter, whose estimated radiation dose is 357 rad and 365 rad respectively. In CFU-F of both cases, stable types of chromosome aberrations were detected. The rate of these chromosome aberrations related well to the chromosome aberration rate of peripheral T -lymphocytes of the same case. These findings suggest the possibility that chromosome aberration exists in somatic cells other than hematopoietic cells of heavily exposed atomic bomb survivors.