AML1/MTG8 was quantified relative to the expression of the GAPDH housekeeping gene by real-time RT-PCR in 22 patients with t(8;21)-positive acute myeloblastic leukaemia (AML) at initial diagnosis and in seven of these patients also during/after chemotherapy and allogeneic bone marrow transplantation. Real-time PCR was able to specifically detect and quantify AML1/MTG8 over a 5 log range. The detection limit for t(8;21)-positive cells was a dilution of 1:105. The AML1/MTG8 expression varied considerably among the 22 AML patients at intial diagnosis with a ratio AML1/MTG8:GAPDH of 0.5135+/-0.536 (range 0.1-2.14, median 0.318). In six patients with t(8;21)-positive AML a marked decline of AML1/MTG8 could be induced by chemotherapy. These patients are in ongoing complete haematological remission (CR) with a constant low-level AML1/MTG8 expression. In another patient a rapid rise of AML1/MTG8 transcripts could be detected in CR after allogeneic bone marrow transplantation and the patient relapsed 10 weeks later. In conclusion, real-time RT-PCR is a suitable approach for the quantification of AML1/MTG8 transcripts in the monitoring of AML patients with t(8;21) during/after chemotherapy and can provide data of prognostic relevance.