All karyotypes of unstimulated blood leukocytes from a young adult patient with myelofibrosis myeloid metaplasia contained 2 small extra metacentric chromosomes which may have originated in a leukocyte stem cell line by long-arm breakage in chromosome 18.Aneuploidy of the C group, F group, and Y chromosome was also found in some unstimulated leukocytes. Karyo types of PHA-stimulated blood lymphocytes and skin fibroblasts were normal.