The 8;21 translocation is one of the most common chromosomal translocations in acute myelogenous leukemia (AML), accounting for 40% of pediatric AML with French-American-British (FAB)-M2 morphology. The chromosomal breakpoints have recently been identified at the molecular level and shown to involve the AML1 gene on chromosome 21 and the ETO gene on chromosome 8. Translocation results in the consistent fusion of these genes on the der(8) chromosome, resulting in the production of a novel chimeric gene and message. Using oligonucleotide primers derived from the AML1 and ETO cDNAs, we were able to amplify a specific fusion transcript from 26 of 26 patients with t(8;21) by a reverse transcriptase polymerase chain reaction (PCR) approach. DNA fragments of identical size were generated from each case including two with complex translocations. Studies on the sensitivity and specificity of this approach show that PCR analysis can be used as a rapid, accurate, and sensitive means for detecting this chromosomal abnormality, and for following the patients' response to therapy.