We evaluated retrospectively the cryptic t(12;21)(p13;q22) in 15 children with early B-lineage acute lymphocytic leukemia who had a normal karyotype by using the locus specific probes of TEL and AML1 genes in a dual color fluorescence in situ hybridization (FISH). The FISH analysis revealed six patients with the fusion gene TEL/AML1 on chromosome 21, three of whom possessed a double fusion gene. In addition, the AML1 probe revealed hyperdiploid clones that were not detected in the conventional cytogenetic analysis. A discrepancy between the proportion of cells with the fusion gene in interphase nuclei and metaphases was noted.