Patients with myelodysplastic syndrome (MDS) and patients with acute myelogenous leukemia (AML) share certain specific karyotypes. Therefore, we compared the relative importance of karyotype and morphology (MDS vs. AML) in determining survival in these patients. We analyzed 768 patients with a banded karyotype who presented as either MDS or AML. Patients with "unfavorable" karyotypes had similarly short survivals regardless of whether they were classified as refractory anemia with excess blasts (RAEB), refractory anemia with excess blasts in transformation (RAEBT), or AML. Patients with diploid karyotypes survived significantly longer, again with relatively minor differences between patients with RAEB, RAEBT, and AML. We conclude that classification of patients with excess myeloblasts in the marrow might more appropriately be based on cytogenetics than on the distinction between MDS and AML.