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 < 0.005). It is supposed that atypical additional abnormalities reflect relatively late stages of leukemia, and their presence before therapy predicts poor prognosis. In fact, general, relapse-free, and uneventful survival rates in patients with atypical abnormalities were significantly lower that in the remaining patients with t(8;21) AML. Poor survival was associated not only with early relapses but also with high mortality from fatal infections soon after onset of treatment. The incidence of fatal infections in this group was significantly higher than in patients without atypical abnormalities (p = 0.027). Atypical additional abnormalities are rather variable and each variant should to be specifically characterized to estimate its prognostic significance. Our results need to be verified in a larger-scale multicentre study.