This paper describes a family in which seven members, involving three consecutive generations, suffered from acute myeloid leukemia during the past 16 years. The genetic abnormalities were vertically transmitted in a Mendelian dominant manner without sex linkage. Cytogenetic approaches, such as G-banding karyotypes, micronuclei (MN), chromosome aberrations (CA), fragile sites (Fra), cell cycle time (Tc), sister chromatid exchanges (SCE), silver-staining nucleolar organizer regions (Ag-NOR), and silver-staining acrocentric chromosome satellite association (Ag-AA), were investigated on 19 presently healthy members of the family, compared with 10 normal controls. The results showed that their G-banding chromosome karyotypes were normal, without a single similarly located fragile site being found to be carried commonly by the blood relations, although the rare fragile site frequency in the blood relations group was higher than that in the non-blood relations group or normal controls. On the other hand, SCE and Ag-NOR were lowered in the blood relations. In III-13 and IV-3 of the pedigree, in particular, the prolonged cell cycle time with distinctly abnormal SCE and Ag-NOR might predict a high risk for leukemia. Hence the follow-up of this remarkable family is being continued.