Background: Acute leukemia (AL) in infants generally shows distinctive biologic features and has a poor prognosis. Aim: To study the frequency of the cytogenetic alteration of11q23 chromosome or the recombination of MLL gene in infants less than 18 months old, with acute leukemia. Patients and methods: We analyzed 37 cases of AL in infants less than 18 months of age diagnosed in Chile from 1989 to 1999. The clinical features and cytogenetic/molecular defects of 11q23MLL gene rearrangement and their influence in prognosis were determined. Results: There were 18 cases of acute Lymphoblastic leukemia (ALL) characterized by female sex (67%) high presenting leukocyte count (median 99 x109/L), blast cells with a CD10 negative phenotype (50%) and 11q23/MLL rearrangement (39%). Molecular abnormalities of 11q23 were significantly associated with adverse prognosis, with an event free survival (EFS) of only 14 ± 12%. Interestingly, infants with germ line 11q23 had a very good outcome with an EFS of 73 ± 11% (p<0.025). There were 19 cases of acute myeloblastic leukemia (AML) characterized by male sex (63%) high leukocyte count (median 93 x 109/L), FAB-MS morphology (53%) and 11q23/MLL rearrangement (53%). EFS was very poor, 20 ± 9% and 33±4% for rearranged and germinal group respectively (p=NS), due to a high mortality rate during the first month of diagnosis. Conclusions: These findings demonstrate that Chilean ALL infants with 11q23 abnormalities have a very poor prognosis. However those with germinal state can enjoy a prolonged disease free survival with the current treatment protocols (Rev Méd Chile2001; 129: 634-642)