BACKGROUND AND OBJECTIVE: Patients with AML, RAEB-t, or RAEB and abnormalities involving chromosomes 5 and/or 7 (-5, -7) generally, but not always, have poorer prognoses than patients with a normal karyotype. Our objective was to see whether the occasional relatively favorable outcome in -5/-7 patients is a random event or, rather, reflects true heterogeneity in -5/-7. DESIGN AND METHODS: We examined 3 factors known to be prognostic in AML for their prognostic significance in 400 -5/-7 patients treated at the M.D. Anderson Cancer Center from 1980-1998 for AML or MDS. The outcome of comparative interest was survival as assessed by log-rank test. RESULTS: There was evidence that outcome was better in -5/-7 patients with a simple (rather than complex) karyotype, with > 1 normal metaphase (rather than only metaphases containing -5/-7), and without an antecedent hematologic disorder. More importantly, the 10% of the patients with a simple karyotype, > 1 normal metaphase, and no antecedent hematologic disorder not only had a better outcome than the other -5/-7 patients but had essentially identical outcomes to the 669 AML/MDS patients with a normal karyotype treated at M.D. Anderson during the same period. INTERPRETATION AND CONCLUSIONS: The results indicate that the -5/-7 group should not a priori be regarded as having an unfavorable prognosis, and more generally suggest the need to refine prognosis within each of the cytogenetic subsets of AML.