Three new cases are reported of cytogenetically Philadelphia-negative (Ph-) chronic myelocytic leukemia (CML), with positive BCR/ABL gene rearrangement according to a reverse transcriptase polymerase chain reaction technique. Fluorescence in situ hybridization (FISH) studies using different probes showed three different situations involving chromosomes 9 and 22 for the masked BCR/ABL fusion gene. With the use of BCR/ABL-extra signal and CEP 9 probes (Vysis, Downers Grove, IL, USA), FISH studies detected the BCR/ABL fusion gene at the end of chromosome 9 in patient 1, a BCR/ABL fusion gene on both chromosomes 22 in patient 2 (who was in an accelerated phase of CML), and a BCR/ABL fusion signal on chromosome 22 in patient 3. Interestingly, FISH interphase signals showed the same pattern in patients 1 and 3, but the BCR/ABL fusion gene was located on different chromosomes. Careful interpretation of the results and a simultaneous study of nuclei and metaphases are therefore recommended in each case. In conclusion, in cases of Ph- CML, FISH studies are of paramount importance since they can detect chromosomal reorganization and its location, and can also provide quantitative follow-up of these patients.