Recently, a new recurrent translocation, t(12;21)(p13;q22), has been identified in B-cell lineage acute lymphoblastic leukemia (ALL). The translocation results in the fusion of two known genes, ETV6/TEL(12p13) and AML1(21q22), both of which have been shown to be involved in other hematological malignancies. The t(12;21) is virtually undetectable by routine cytogenetics, but the chimeric transcript ETV6-AML1 has been detected in childhood ALL by molecular techniques in up to 36% of cases, making it the most common genetic abnormality in these patients. It has been shown to be associated with a B-precursor phenotype and an excellent prognosis. We tested 66 diagnostic pediatric ALL samples by reverse transcription polymerase chain reaction (RT-PCR) and found evidence of the t(12;21) in 22 (33%). None of these had previously been identified as harboring the t(12;21), although six had karyotypic abnormalities involving either 12p13 or 21q22. ETV6-AML1 expression defined a subgroup of patients characterised by an age of between two and 12 years, B-lineage immunophenotype and non-hyperdiploid DNA content. Our data further support the importance of molecular diagnostic methods in the identification of clinically distinct subgroups of patients with ALL.