Quantification of AML1-MTG8 fusion transcripts was performed by using real-time reverse transcription-polymerase chain reaction (RT-PCR) and the clinical value of this method was evaluated in t(8;21)-positive acute myelogenous leukemia (AML). A t(8;21)-positive cell line, Kasumi-1, was used for constructing standard curves and the corrected AML1-MTG8 mRNA expression level relative to the expression of the GAPDH housekeeping gene was calculated. Bone marrow samples from 14 patients with t(8;21)-positive AML were sequentially examined. The corrected AML1-MTG8 expression level at diagnosis varied in the range from 0.4 to 2.7 (median, 1.5) among the patients. When samples at 1, 3 and 6 months were examined after diagnosis, the corrected AML1-MTG8 expression level was found to decrease sequentially in all but one. AML1-MTG8 fusion transcripts were also detected in four of eight samples from patients in remission for more than 1 year. In conclusion, real-time RT-PCR can provide a rapid and accurate quantification of AML1-MTG8 fusion transcripts. This system could be useful to reveal the prognostic relevance of minimal residual disease in t(8;21)-positive AML.