A review of patients with myeloid disorders presenting to a large cytogenetic referral centre over a ten year period was undertaken to assess the clinical relevance of the presence of del(20q) in their malignant karyotypes. Twenty-six patients were identified, four with myeloproliferative disorders (MPD), 15 with myelodysplastic syndromes (MDS) and seven with acute leukemia. The presence of del(20q) in four patients with MPD did not appear to adversely affect survival, with all patients alive 18 to 184 months post diagnosis. However, the 15 patients with MDS had a median survival of only 12 months. Seven of these patients developed acute leukemia including three of four patients with refractory anemia with ringed sideroblasts (RARS). Of the seven patients with acute leukemia de novo and del(20q), six were treated with only two achieving complete remission. The median duration of survival for these seven patients was 5 months. These results, when compared with published survival data from the MIC Cooperative Group, indicated that del(20q) in MDS is associated with a high rate of transformation to acute leukemia and a poor prognosis. In de novo acute leukemia, del(20q) is associated with a poor response to treatment and reduced survival.