The t(4;11)(q21;q23) chromosomal abnormality was identified in 40 (2%) of 1,986 children with newly diagnosed acute lymphoblastic leukemia (ALL). This translocation was associated with female sex (63%), age less than 1 year (60%), hyperleukocytosis (median leukocyte count, 156.5 x 10(9)/L), CD10-/CD19+ B-precursor cell immunophenotype, and myeloid-associated antigen (CD15) expression (63%). Nearly all cases had at least some CD24- blast cells. The CD10-/CD15%/CD19+/CD24/+ phenotype was found in 20 of the 32 t(4;11) cases tested. None of the 40 cases had the cytogenetic finding of hyperdiploidy greater than 50, which is a favorable prognostic feature. For clinical comparison, the t(4;11) cases were divided into three groups according to age at diagnosis: less than 1 year (n = 24), 1 to 9 years (n = 8), and greater than or equal to 10 years (n = 8). Compared with older patients, infants were more likely to have initial central nervous system leukemia (P = .05) and less likely to have pre-B-cell ALL (P = .05). Complete continuous remission has been maintained in only 7 of 24 infants and 2 of 8 patients aged greater than or equal to 10 years, in contrast to 7 of 8 children in the intermediate age group (P = .048). These findings suggest that the t(4;11) is an adverse prognostic feature in these two age groups.