To study the pathogenetic significance of acquired del(20q), bone marrow morphology and clinical histories of 107 patients with hematologic disorders and the del(20q) were studied. In 84 cases representing myelodysplastic syndromes (47), myeloproliferative disorders (31), acute myeloid leukemia (3), pure red blood cell aplasia (2), and angioimmunoblastic lymphadenopathy with dysproteinemia (AILD-like T-cell lymphoma (1), 20q- was the sole karyotypic abnormality. The breakpoints on chromosome 20 were not constant, and in 77% of cases, only a fraction of the studied metaphases had del(20q). In 23 cases, including myelodysplastic syndromes (13), myeloproliferative disorders (6), acute myeloid leukemia (2), and autoimmune disorders (2), 20q- occurred with other cytogenetic abnormalities, including del(5q), -7, +8, 13q deletion or translocation, and +21. Despite the diagnostic heterogeneity, the bone marrow morphologic abnormalities consistently involved the erythroid precursors and megakaryocytes. 20q- is a primary cytogenetic abnormality occurring in hematologic disorders unified morphologically by dysplasia in erythroid precursors and megakaryocytes. In conjunction with other studies of disorders involving del(20q) or genes on the long arm of chromosome 20, the findings suggest that del(20q) disproportionately affects a common megakaryocytic/RBC stem cell.