Acute lymphoblastic leukemia (ALL) is associated with recurrent chromosomal abnormalities. The cryptic translocation t(12;21)(p13;q22), which leads to the TEL-AML1 fusion gene, is the most common abnormality in childhood B-cell ALL. The aim of this retrospective study was to determine the incidence of TEL-AML1 fusion in childhood and adult B-cell ALL using interphase fluorescence in situ hybridization (I-FISH) and its association with additional changes. FISH, using dual-color extra-signal (ES) DNA probe specific for the TEL and AML1 genes, was performed either on blast cells suspensions stored in liquid nitrogen immediately after Ficoll or on leukemic cells preserved in fixative solution at -20 degrees C after short-term culture. No TEL-AML1 fusion was observed in the 26 ALL adults. The fusion was found among 19.6% of the 57 ALL children, additional changes being identified by conventional cytogenetics in 80% of the cases. A deletion of the untranslocated TEL was observed in 36.3% of the ALL with the TEL-AML1 fusion. The coexpression of myelocytic and B-lymphoid antigens was found in 3 of the 11 of TEL-AML1 fusion positive-ALL. Our results (frequency of TEL-AML1 fusion in children and of the deletion of the untranslocated TEL allele, mean age of the patients and white blood cell count) are within the range observed by others. Structural chromosomal abnormalities other than the t(12;21) are frequent and may play a role in the prognosis of these patients.