The authors studied clinical and immunologic characteristics of six children with 1;19 translocation-associated acute lymphoblastic leukemia (ALL); two of the six had the balanced type, t(1;19)(q23;p13), three had the unbalanced type, -19,+der(19)t(1;19)(q23;p13) with a resultant partial 1q trisomy, and the other had a mosaicism of cells with the balanced type and those with the unbalanced one, t(1;19)/-19,+der(19)t(1;19). Leukemic cells of all three patients, in which intracytoplasmic immunoglobulin was determined, were proved to show pre-B phenotype. Of the six patients, three had an initial leukocyte count of greater than 50 X 10(9)/l, and were classified in the high-risk group. Three patients relapsed after a brief remission and died. The mosaicism observed suggested that at least in some patients the leukemic cells with -19,+der(19)t(1;19) might derive from those with t(1;19) as a step in the course of clonal evolution. Our data and a review of the literature indicate that there may be no differences in the clinical and immunologic characteristics between the patients with the balanced translocation and those with the unbalanced one, and that the leukemia with the 1;19 translocation may join with other translocation-associated ALLs, with which the patients are known to have poor prognosis.