Abstract We investigated 20 patients with hematologic disorders who had a clone of cells with a deletion of most of a chromosome #20 long arm (20q−) in the bone marrow. Three patients had polycythemia vera (PV), 6 had acute nonlymphocytic leukemia (ANLL), 8 had preleukemia (PL), and 3 had other myeloproliferative disorders. In our laboratory, a 20q− chromosome is found in 6% of patients with PV, 3% of patients with ANLL, and 1% of patients with PL. Among the 6 patients with ANLL and a 20q− abnormality, 3 had erythroleukemia. There were no apparent clinical differences among our patients with 20q− chromosomes compared with other patients with similar disorders who did not have a 20q− chromosome. The breakpoint of the 20q− anomaly, in each instance, was in band 20q11, but it occurred near the centromere at 20q1101 in 16 patients and at the distal part of this band at 20q1109 in 4 patients. Three of the 4 patients with a breakpoint at 20q1109 had PL.