The molecular approach for the analysis of leukemia associated chromosomal translocations has led to the identification of prognostic relevant subgroups. In pediatric acute lymphoblastic leukemia (ALL), the most common translocations, t(9;22) and t(4;11), have been associated with a poorer clinical outcome. Recently the TEL gene at chromosome 12p13 and the AML1 gene at chromosome 21q22 were found to be involved in the translocation t(12;21)(p13;q22). By conventional cytogenetics, however, this chromosomal abnormality is barely detectable and occurs in less than 0.05% of childhood ALL. To investigate the frequency of the molecular equivalent of the t(12;21), the TEL/AML1 gene fusion, we have undertaken a prospective screening in the running German Berlin-Frankfurt-Münster (BFM) and Italian Associazione Italiana Ematologia Oncologia Pediatrica (AIEOP) multicenter ALL therapy trials. We have analyzed 334 unselected cases of pediatric ALL patients consecutively referred over a period of 5 and 9 months, respectively. The overall incidence of the t(12;21) in pediatric ALL is 18.9%. The 63 cases positive for the TEL/AML1 chimeric products ranged in age between 1 and 12 years, and all but one showed CD10 and pre-B immunophenotype. Interestingly, one case displayed a pre-pre-B immunophenotype. Among the B-lineage subgroup, the t(12;21) occurs in 22.0% of the cases. Fifteen of 61 (24.6%) cases coexpressed at least two myeloid antigens (CD13, CD33, or CDw65) in more than 20% of the gated blast cells. DNA index was available for 59 of the 63 TEL/AML1 positive cases; a hyperdiploid DNA content (> or = 1.16) was detected in only four patients, being nonhyperdiploid in the remaining 55. Based on this prospective analysis, we retrospectively evaluated the impact of TEL/AML1 in prognosis by identifying the subset of B-lineage ALL children enrolled in the closed German ALL-BFM-90 and Italian ALL-AIEOP-91 protocols who had sufficient material for analysis. A total of 342 children were investigated for the presence of TEL/AML1 fusion gene and 99 cases (28.9%) were positive. The patients expressing the TEL/AML1 fusion mRNA appeared to have a better event-free survival (EFS) than the patients who lacked this chimeric product. Whereas three of the TEL/AML1 positive cases (3.0%) have relapsed to date, 27 patients without TEL/AML1 rearrangement (11.1%) suffered from relapse. To date, the only subset of B-lineage ALL with a favorable prognosis has been the hyperdiploid group (DNA index > or = 1.16 < 1.6). Our findings reinforce the need to include the molecular screening of the t(12;21) translocation within ongoing prospective ALL trials to prove definitively its prognostic impact.