Trisomy 21 is the second most common trisomy in patients with acute myelogenous leukemia (AML) and myelodysplastic syndromes (MDS). However, its clinical and prognostic significance is not known. We analyzed the records of 1187 consecutive patients with untreated AML or MDS. Thirty-seven (3.3%) had trisomy 21: four (0.3%) as the only cytogenetic abnormality and 33 (2.7%) with other cytogenetic abnormalities (-5 and/or -7 in 15, +8 in nine, t(15;17) in three, inv(16) in three, t(8;21) in one, and hyperdiploid with several other additional chromosomes in two). Twenty-eight patients had AML and nine MDS. No patients had megakaryocytic phenotype (M7), common in patients with constitutional trisomy 21 (Down's syndrome) and AML. Overall, 57% achieved complete remission (CR), with median CR duration of 39 weeks, and median survival of 31 weeks. When patients with additional cytogenetic abnormalities were compared to patients with similar abnormalities but no trisomy 21, their clinical features as well as their CR rate, CR duration and survival were similar, with or without trisomy 21. We conclude that trisomy 21 in AML typically presents in conjunction with other cytogenetic abnormalities, especially -5/-7 and +8 whose presence rather than the presence of +21 dictates the clinical outcome.