The occurrence of a missing Y chromosome was investigated in the bone marrow cells of male individuals, i.e., 255 controls, 73 with acute myelocytic leukemia (AML) and 59 with Ph1-positive chronic myelocytic leukemia (CML). The incidence in controls of individuals with 45,X cells increased with age, particularly after the age of 60. In AML, 45,X metaphases were detected in two patients over 70 years of age, but the leukemia seemed to have involved the 46,XY cells rather than the 45,X cells. Four of the six patients with No. 8-No. 21 translocation and two of the 16 with major karyotypic abnormalities (MAKA) exhibited a missing Y in the leukemic cells in addition to other karyotypic aberrations. Four of the Ph1-positive CML patients exhibited a missing Y in all or nearly all the cells in the bone marrow along with the Ph1. In one patient, additional chromosome abnormalities involved the 46,XY,Ph1 rather than the 45,X,Ph1 cells. The genesis of the missing Y in CML cells may be related to the presence of the Ph1, though apparently the patient's age also plays a role. It is our hypothesis that 45,X or 45,X,Ph1 cells are resistant to the development of further chromosomal abnormalities and, thus, reflect their resistance to being involved in an acute leukemic process.