Chromosome studies were made on myeloblasts obtained from the peripheral blood of 4 patients (3 males, 1 female) with acute or chronic granulocytic leukemia and on leukocytes from 3 healthy individuals (1 male, 2 females). The cells were grown in culture for 2 to 4 days before examination. Both in normal and in leukemic cells, the predominant metaphase chromosome number was 46. However, idiogram analyses revealed a definite abnormality, probably involving the Y chromosome, in the 2 cases of chronic granulocytic leukemia. No chromosome abnormality was distinguishable in the 2 cases of acute leukemia (1 male, 1 female) investigated. The results indicate that in some leukemic human leukocytes small but definite chromosomal changes are demonstrable.