Prognostic significance of additional karyotype abnormalities was studied in 73 children with t(8,21) acute myeloid leukemia (AML). Additional chromosomal aberrations were documented in 61 cases (83.6%). The loss of sex chromosomes and/or deletion of the long arm of chromosome 9 (9q-) were predominant abnormalities, in agreement with the literature data. Other additional abnormalities detected in 14 cases were tentatively designated as "atypical". Comparison of pretreatment cytogenetic data and those obtained during relapses revealed the previously unknown rise in the frequency of atypical abnormalities in AML relapses (to 63.6% vs 19.2% at the first presentation, p