Strategies currently used for residual disease detection in acute lymphoblastic leukaemia (ALL) rely on polymerase chain reaction (PCR) detection of immunoglobulin and T-cell receptor rearrangements. The TEL-AML1 fusion transcript, which is associated with t(12;21) (p13;q22), is found in 25% of childhood B-cell precursor ALL, and represents an interesting alternative target. We compared two methods for quantitating TEL-AML1 fusion transcripts: competitive PCR and real-time PCR. These techniques showed similar sensitivity (5 x 10(-5)) and reproducibility. Giving highly correlated results, both techniques can be conveniently used for TEL-AML1 transcript quantification. The constancy of TEL-AML1 expression was evaluated by measuring TEL-AML1 transcripts at different steps of the cell cycle, and in 21 cases of ALL at diagnosis. No major variation in TEL-AML1 expression was observed during the cell cycle or in 20/21 of the ALL patients. Residual disease was then determined after completion of induction therapy in 20 patients with a TEL-AML1-positive ALL. Seven patients out of 20 (35%) were still positive, including two patients with high level of residual blasts (close to or beyond 10(-2)). When comparison was possible, results obtained using TEL-AML1 quantification were in accordance with those obtained using T-cell receptor rearrangements analysis.