Chromosome studies were carried out on 13 cases of various types of leukemias (acute myelogenous and lymphocytic leukemias, monocytic leukemia, erythroleukemia, chronic myelogenous and lymphocytic leukemias, and the Down's syndrome associated with acute lymphocytic leukemia) and on 5 cases of normal controls. In 6 out of the 13 cases, both peripheral blood and bone marrow were studied, and in the other 7 cases only peripheral blood was used. The Ph(1) chromosomes and a rather low diploid modality were observed in either of 2 cases of chronic myelogenous leukemia. No chromosome abnormality except for the trisomy of the 21st chromosome (Denver classification) was observed in a case of the Down's syndrome associated with acute lymphocytic leukemia. In the other types of leukemias, no significant chromosome abnormality was found either in peripheral blood or bone marrow specimens. These findings suggest that in human leukemias there is no consistent chromosome change except for the Ph(1) chromosome in chronic myelogenous leukemia.