The chromosomal breakpoints of t(4;11) translocation of acute lymphoblastic leukemia (ALL) have been recently identified at molecular level and shown to involve the AF4 (FEL) gene on chromosome 4 and the ALL-1 (MLL, Hrx) gene on chromosome 11. The ALL-1/AF4 fusion gene is transcribed into a chimeric mRNA. Using primer sets derived from ALL-1 and AF4 cDNAs by reverse transcription-polymerase chain reaction (RT- PCR), we were able to amplify the breakpoint sites of the fusion transcript of all 15 ALL cases with karyotypic or molecular evidence of the t(4;11). DNA fragments of different size were obtained as the consequence of different breakpoints on chromosome 11 and the presence of alternative splicing of ALL-1 exon 8. The feasibility of monitoring the residual cells carrying the t(4;11) in 2 ALL patients with different clinical outcome was evaluated. Overall, the presented results provide evidence that RT-PCR can be used as a rapid method for detecting this chromosomal abnormality and following the patient's response to therapy.