The fusion transcript AML1/ETO was detected in the bone marrow of two t(8;21)-negative acute myelogenous leukemia (AML) patients by means of reverse transcription-polymerase chain reaction. This fusion transcript is identical to the one transcribed from the t(8;21) translocation base, as deduced from (a) the size and restriction pattern of the amplified DNA fragment and (b) the DNA sequence analysis of the fusion junction. We also showed that the ETO gene is highly expressed in these patients, much as it is in the t(8;21)-positive AML. Southern blot analysis showed rearrangement of the AML1 gene in one of the patients. Together, our results demonstrate that there is a masked t(8;21) translocation in AML that is not detectable by cytogenetic analysis but is able to transcribe an AML1/ETO fusion transcript similar to that transcribed in t(8;21)-positive AML-M2 patients.