Chromosome translocation t(15;17) specifically found in acute promyelocytic leukemia (APL) results in cleavage in the introns of PML gene on chromosome 15 and in the intron of the retinoic acid receptor alpha (RAR alpha) gene on chromosome 17, creation and expression of PML-RAR alpha and RAR alpha-PML fusion genes. Reverse transcription-polymerase chain reaction (RT-PCR) was applied to detect the PML-RAR alpha fusion transcripts rapidly in APL patients. The fusion transcripts could be detected in all of the 10 APL patients studied. Of the two breakpoints in the PML gene so far reported, seven APL patients had the fusion transcript compatible with the downstream (3') breakpoint, and the other three APL patients were considered to have the upstream (5') breakpoint. RT-PCR could detect the fusion transcripts from as little as 50 pg bone marrow RNA, and from as little as 0.5 pg bone marrow RNA with the nested PCR. This method was applied to detect minimal residual leukemia cells in an APL patient who had undergone allogeneic bone marrow transplantation, in whom the RT-PCR could not detect the PML-RAR alpha fusion transcripts at several post-transplant time points. This system could be useful to detect minimal residual leukemia cells and accordingly modify the treatment strategy, as well as to make a quick diagnosis with a small amount of clinical sample.