The whole process of a second case of chronic granulocytic leukemia in Nishiyama district where a very small amount of radiation existed for a long time was reported together with data measured by a human counter and the results of chromosomal analysis. No significantly high K or /sup 137/Cs values were measured by a human counter immediately after the onset. Chromosomal division aberration and chromosomal aberration, which seemed to be induced by radiation, also were not observed. However, granulocytic leukemia was diagnosed after chromosomal analysis of peripheral blood revealed Ph/sup 1/ chromosomes, white cell count increased, juvenile cells appeared, and basophil cells increased. Clinical features of typical chronic granulocytic leukemia in the exposed were observed during the chronic stage (7 years). In the acute stage, abnormal clones were discovered in all 16 chromosomes analyzed. Much karyotypic evolution identical to that in persons directly exposed to the A-bomb was also observed.