This report describes a chronic myelogenous leukaemia patient with an apparently normal bone marrow karyotype but BCR/ABL fusion-gene-positive. Commercial FISH probes showed an atypical pattern and the BCR/ABL fusion transcript was detected by RT-PCR, but not the reciprocal ABL/BCR. Consecutive FISH assays clarified the mechanism of the masked Ph. The ABL gene and the following 5.6-5.7 Mb of 9q are inserted into the BCR region of chromosome 22. There is no transference of 22q material to chromosome 9 or to any other chromosomes. Clinical features and evolution of the patient are similar to those cases with classic Ph chromosome.