About one half of Philadelphia (Ph)-positive acute leukaemia (AL) patients and rare cases of Ph-positive chronic myelogenous leukaemia (CML) show rearrangements within the first intron of the BCR gene on chromosome 22. We studied breakpoints within the first BCR intron in 22 adult patients with Ph-positive leukaemia; 21 with AL and one with CML, which lacked rearrangements within the major bcr (M-bcr). With a series of genomic probes from this intronic region, we detected chromosomal breaks in all 22 patients within the 35 kb area, corresponding almost to the 3' half portion of the intron. The breakpoints were distributed throughout this region but we could not identify any special cluster of breakpoints in this area. Our data support consistent involvement of the 3' half part of the first BCR intron in Ph-positive leukaemias without M-bcr rearrangement, and indicate relatively wide scattering of breakpoints in this portion of the intron.