The t(12;21) produces the gene fusion ETV6/RUNX1 and is a frequent rearrangement in childhood ALL, associated with a good prognosis. In Mexico its prevalence has not been reported. This study evaluated a group of consecutive Mexican children with newly diagnosed ALL, to detect the fusion using fluorescence in situ hybridization (FISH). Seventy-one bone marrow samples were analyzed with FISH, using ETV6/RUNX1 DNA probes. Abnormalities of ETV6, RUNX1, or both were found in 31 of the 71 (44%) patients. Six showed ETV6/RUNX1 fusion and 17, with extra RUNX1 copies, presented an additional chromosome 21 or dup(21)(q22). Five patients had structural changes in ETV6, and three patients showed extra copies of ETV6 and RUNX1 from polysomy of chromosomes 12 and 21. Our results revealed a fusion in 8.5% of the 71 cases analyzed. This frequency is lower than that observed in other populations (9.5-32%). The structural rearrangements resulting in RUNX1 extra copies were found in 9.8% of patients, which is close to the range reported (1.5-9.7%) by other authors. Due to the prevalence of RUNX1 overrepresentation in our population and its unknown prognostic significance, further studies should be conducted in consecutive children with ALL, to correlate this abnormality with the patients' follow-up.