BACKGROUND: TEL (ETV6)/AML1 (RUNX1) rearrangement is observed in approximately 20-25% of childhood precursor B-ALL and is associated with a favorable outcome. Additional genetic changes, associated with TEL/AML1, are frequently found. We evaluated the prevalence and prognostic significance of TEL/AML1 rearrangement and additional genetic changes in the TEL and AML1 genes in Korean childhood precursor B-ALL. METHODS: We performed FISH using LSITEL/AML1 ES probe (Vysis, USA) in 123 children diagnosed as having precursor B-ALL and assessed clinical relevance of the TEL/AML1 rearrangement and additional genetic abnormalities. RESULTS: The frequency of TEL/AML1 was 17.1% (21/123) in patients with precursor B-ALL. TEL/ AML1-positive group showed male predominance (P=0.012) and younger age of onset than TEL/ AML1-negative group by 1.6 yr (P=0.013). The outcome of TEL/AML1-positive group tended to show lower incidences of relapse (1/21 vs 20/102), death (1/21 vs 17/102) and longer event free survival. Among TEL/AML1-positive patients, unrearranged TEL deletion, AML1 gain, and unrearranged TEL deletion combined with AML1 gain were detected in 61.9%, 23.8%, and 9.5%, respectively. There were no significant differences in the clinical features and outcome according to the presence or absence of additional genetic changes. CONCLUSIONS: The frequency of TEL/AML1 and additional genetic changes in TEL and AML1 is higher than previous studies in Korean children, and in close agreement with usually reported one, 20-25%. TEL/AML1-positive group showed a tendency toward better prognosis. Further study is needed to clarify the prognostic significance of additional changes in TEL and AML1 based on a large sample size.