We used fluorescence in situ hybridization (FISH) to metaphase chromosomes with BCR and ABL cosmid probes in conjunction with the polymerase chain reaction (PCR) to study the mechanism by which the ABL proto-oncogene is inserted into a morphologically normal chromosome 22 in patients with Ph-negative chronic myeloid leukaemia characterized by the BCR-ABL chimeric gene. In control patients with Ph-positive CML the ABL probe localized to 22q- and the 3' BCR probe localized to 9q+. In nine Ph-negative CML patients the ABL probe localized to one normal chromosome 9 and to one 'normal' chromosome 22. Both 5' and 3' BCR probes localized exclusively to the chromosomes 22. By PCR all had evidence of BCR-ABL transcripts, but none had evidence of the reciprocal ABL-BCR gene product that is seen in 70% of the Ph-positive CML patients. These data confirm the view that Ph-negative CML results from insertion of ABL-containing DNA sequences into a normal-appearing chromosome 22 without reciprocal translocation of sequences from chromosome 22 to chromosome 9.