Recently it has been shown that t(12;21) (p13;q22) fuses two genes TEL and AML1 previously cloned from translocation breakpoints in acute myeloblastic leukemias (Blood 1995, 85, 3662). Because classical banding cytogenetic techniques regularly miss t(12;21), RT-PCR molecular detection was used in all our patients (pts). The chimeric transcript is detectable in 23% of childhood B cell precursor ALL. We have studied clinical and biological manifestations and outcome in 17 ALL children with t(12;21) treated in our center according to protocol FRALLE 93 and compared them to the other pts. The results are: mean age 6.1 y (2 - 16), mean Hb 7.4 gr% (3 - 10.2), mean platelets 87000 mm3 (9000 - 300000), mean WBC 25000 mm3 (2500 - 153000), tumor burden 23%, L1/L2 10/7. Karyotypes are: failure: 3, normal: 9, tetraploid: 1, 12 p- abnormalities: 3, 1q and 4p abnormality: 1. There were no clinical statistical differences in clinical manifestations among the pts with the translocation as compared to other pts with B lineage childhood ALL but myeloid antigens (CD13 and/or CD33) were present in 68% of cases. Distribution of patients in low, intermediate and high risk groups were respectively 8%, 70%, 23%, not different from other BALL pts (13%, 49%, 38%). All pts were good steroid responders, without blasts at bone marrow examination on day 21 and achieved CR within a month. All but one pt are in CR with a mean follow up of 18 m (3 - 36). One pt with tetraploid karyotype died from interstitial pneumonia after BMT, performed in first CR. Conclusion: TEL-AML1 is the most common gene rearrangement in childhood ALL despite the t(12;21) not being cytogenetically evident. There are no significant findings in pts with t(12;21) other than a higher frequency of myeloid associated antigens. The better outcome of these pts seems to be related to the translocation but a longer follow up is necessary.