Lymphoblasts from children with B-progenitor cell acute lymphoblastic leukemia (BpALL) with chromosomal hyperdiploidy and with translocations affecting chromosome 12p11-13, accumulate high and low levels of methotrexate polyglutamates (MTXPGs), respectively. Recently a cryptic translocation, t(12;21) (p13;q22), has been demonstrated by molecular and fluorescence in situ hybridization techniques in this disease. The chimeric TEL-AML1 transcript, which has been associated with this translocation, can be detected in up to 25% of children with BpALL. We detected the TEL-AML1 and/or the AML1-TEL transcript in 30 (33%) of 91 patients studied. Levels of lymphoblast MTXPGs were lower in those with than in those without the TEL-AML1 translocation (P = 0.004). Hyperdiploidy was rare in lymphoblasts with the TEL-AML1 translocation (P = 0.047). Both ploidy (P= 0.0015) and TEL-AML1 status (P= 0.0043) were independently and significantly correlated with the log of the lymphoblast MTXPG level. However, the presence of TEL-AML1 or of hyperdiploidy accounted for only 22% of the variation of this value. Our results imply that each of 1.16 > or = DI and the presence of the TEL-AML1 translocation confers a 50% decrease in lymphoblast MTXPG level. When planning reduction of therapy for either of the two excellent outcome categories of hyperdiploid or TEL-AML1 BpALL, one should consider the difference between these two subgroups in the ability of lymphoblasts to accumulate MTXPGs.