A chromosomal abnormality in marrow and blood cells has been found in only one patient out of a group of 20 subjects with myeloproliferative disorders other than leukemia. The abnormal karyotypic finding consisted of group C9 trisomy in a patient with myeloid metaplasia and an acute leukemia-like picture and indicates a definite relationship to acute leukemia. The latter has been shown to be not infrequently accompanied by C9 trisomy. The trisomy was accompanied by the presence of a substantial number of hypertetraploid cells in the marrow but not in the cultured blood cells. As a matter of fact, the blood culture yielded predominantly metaphases with 47 chromosomes (C9 trisomy) on the first examination and metaphases with 46 chromosomes and a normal female karyotype on the second occasion. The superiority of bone marrow karyotype analysis over that of blood cells in leukemic states is thus indicated.