Detailed clinical and cytogenetic studies were performed in five patients who had abnormal hematopoiesis and an acquired deletion of an F-group chromosome. Cytogenetic analyses, with banding techniques, of cells from bone marrow, spleen, or unstimulated peripheral blood showed a partial deletion of the long arm of one chromosome 20 [del(20)(q11)] in all five patients. Three patients had myeloproliferative disorders of uncertain classification, the fourth had possible preleukemia, and the fifth had acute myelomonocytic leukemia. Although the five cases showed certain similarities, the clinical and hematologic findings seen with the 20q- abnormality were not specific. None of the patients showed evidence of polycythemia vera or idiopathic acquired refractory sideroblastic anemia, two diseases previously associated with the 20q-. Our studies indicate that the 20q- abnormality is not limited to diseases primarily affecting erythropoiesis but that it can be found in the broader spectrum of myeloid disorders. In polycythemia vera, the 20q- has sometimes been regarded as a possible result of previous therapy with cytotoxic agents; however, four of our patients were untreated when the deletion was first noted.