The persistence of the AML1-ETO rearrangement performed by reverse transcription polymerase chain reaction (RT-PCR) has been reported in acute myeloid leukemia (AML) patients in long-term complete remission (CR). This persistence, which is not associated with hematological relapse, limits the clinical use of qualitative RT-PCR. Here, we present a new quantitative real-time PCR method to detect AML1-ETO rearrangement using fluorescently labeled probes. Quantitative detection of AML1-ETO was performed in capillary tubes using two fluorescently labeled probes in the LightCycler equipment. The reliability of the method was checked in twenty-two bone marrow samples and one apheresis sample from eight patients with t(8;21) collected at diagnosis and during follow-up assessment. The regression coefficients obtained for standard curves of AML1-ETO and AML were all greater than 0.98. The sensitivity attained allowed the detection of rearrangements at a dilution of 10(-5) Kasumi-1 cDNA. The intra-assay coefficient of variation was 4% for AML1-ETO, and 7% for AML. The inter-assay coefficient of variation was 19% for AML1-ETO and 12% for AML. A log reduction from two to four in the AML1-ETO/AML ratio was evident after CR. The study of the method and first results obtained in patient samples support that quantitative real-time PCR with hybridization probes is a new reliable and sensitive method to monitor minimal residual disease in AML patients. Moreover, the fluorescent probes with the Light-Cycler technology offer the advantage of a rapid detection.