There is little information about the breakpoint cluster regions of the BCR/ABL fusion gene in Mexican Mestizos with Philadelphia chromosome positive chronic myelogenous leukemia; in a small study a different distribution of these as compared with Caucasians was recently described. We have now prospectively analyzed the breakpoint cluster regions of the BCR/ABL fusion gene in a group of 238 Mexican Mestizos patients with Philadelphia chromosome positive chronic myelogenous leukemia and found a prevalence of 54.2% for the b3/a2 subtype, 43.2% for the b2a2 and 2.5% for the b3a2/b2a2. These data are not different from those previously described in other populations and are consonant with the prevalence of chronic myelogenous leukemia in Mexico, which is not different from that described in other populations.