The translocation between chromosomes 8 and 21, t(8;21) (q22;q22), is the most frequent abnormality in acute myeloid leukemia (AML) with French-American-British type M2 (FAB-M2) morphology. The breakpoints in this translocation have been characterized at the molecular level, and the genes involved are AML1 on chromosome 21 and ETO on chromosome 8. The rearrangement of the two chromosomes results in a fusion gene and in the production of a consistent fusion transcript on the der(8) chromosome. We have used oligonucleotide primers derived from both sides of the fusion cDNA junction and reverse transcription-polymerase chain reaction (RT-PCR) to analyze six AML-M2 patients with a t(8;21) during various stages of their disease. Two patients studied at diagnosis and one studied at first relapse are alive off therapy and in continuous complete remission for 83 to 94 months. We have detected the AML/ETO fusion transcript in recent peripheral blood samples from each of them. Three other patients also had a fusion transcript detected after 1 to 4 months in remission. Two of these patients subsequently relapsed and died whereas the third patient is alive and in continuous complete remission 70 months later. Thus, our preliminary data suggest that cells with the translocation are still circulating in t(8;21) patients in long-term remission. This finding raises serious questions regarding the interpretation of positive results obtained only with this technique that may not be suitable to decide appropriate further treatment for patients in clinical remission.