The chromosomal abnormality in chronic myeloid leukemia (CML) results from a reciprocal translocation between chromosomes 9 and 22 transferring the c-abl proto-oncogene from chromosome 9 to the restricted breakpoint region on chromosome 22 M (bcr). In this study the breakpoint was determined within the M-bcr in 35 CML patients in the chronic phase, by Southern blotting analysis, and it was then correlated with bone marrow Granulocytic-Megakaryocytic (GRAN-MEG) and Granulocytic (GRAN) histological subgroups, as well as with the clinical findings and laboratory parameters. In the 35 patients analyzed, 46% were grouped as 5' and 54% as 3'. There was an increase in bone marrow basophils in 5' breakpoint patients compared to 3' breakpoint (p = 0.042) but the M-bcr breakpoint site did not differ significantly in the subgroup GRAN or GRAN-MEG (p = 0. 12). In conclusion, the patient population had a higher frequency of M-bcr breakpoint in zone 4 and 3' position; there was no correlation between 5' and 3' positions and clinical or haematological features, except a significant increase in bone marrow basophil cells in 5' breakpoint patients compared to 3' breakpoint. Although a higher frequency of the 3' breakpoint was found in patients with a low number of megakaryocytes compared to the cases with a granulocytic-megakaryocytic proliferation, this difference was not statistically significant.