Acute lymphoblastic leukemia (ALL) is the most frequent childhood cancer. The leukemic cells of ALL patients show several well defined numeric and structural chromosomal abnormalities which are universally known for its prognostic implications. We studied a group of 44 children with ALL, to investigate the incidence of chromosome aberrations in ALL, its lymphocyte lineage and some clinical feature associations, and the finding of non previously described aberrations. A high proportion of patients (79.5%) showed chromosomal abnormalities. Most of them had a pseudodiploid karyotype (46 chromosomes), characterized mainly by a translocation. In relation to chromosome number, 27% of them were hyperdiploid with more than 50; 9% hyperdiploid between 47-50 and 7% hypodiploid (less than 46). Among structural aberrations found, were the following recurrent translocations: t (1; 19), t (4; 11), t (9; 22) in 6.8%, 9.1% and 2.3% of cases respectively, all related to an early B immunophenotype. Other translocations found, compromised regions 7q22, 9p21 - 24. Two new translocations in ALL were found: t (1; 5)(q23; q33), apparently balanced, and t (13; 21)(q14; q22), unbalanced. Other recurrent structural changes found were: deletion (6q), (7q), (9p), (11q), (12p), inversion (3q), isochromosome (7q), maker chromosomes and double minutes. The distribution of chromosome abnormalities in this group of patients was in agreement with previous reports from other investigators.