Chromosome banding studies on 60 children with acute lymphocytic leukemia (ALL), including “null,” pre-B, B, and T cell phenotypes, were performed. In 4 of 17 patients with pre-B cell ALL, we noted a previously undescribed chromosome translocation, t(1;19)(q23;q13). This translocation was not found in patients with “null” cell, B cell, or T cell ALL. Since each patient with the 1;19 translocation experienced early treatment failure, t(1;19)(q23;q13) may mark a subgroup of patients with pre-B cell ALL who have an especially poor prognosis.