A case in which C monosomy of hematopoietic elements, with myelofibrosis, was found in a 5-year-old Negro boy is presented. The pathologic process may represent a disease which can present at any stage in the myeloproliferative spectrum and advance to acute myelomonocytic leukemia. Chromosomal studies with and without phytohemagglutinin are stressed. C monosomy myeloproliferative disease is suggested as the name of the disease process.