We studied 210 unselected patients with acute myeloid leukaemia (AML) for MLL abnormalities. Twenty-seven patients (13%) with rearranged MLL genes were identified by means of Southern blot analysis. An MLL-AF6 fusion transcript was detected in six patients by a reverse transcriptase polymerase chain reaction (RT-PCR) for the MLL-AF6 translocation. Sequence analysis showed fusion of MLL exon 7 as well as exon 6 (two patients) or MLL exon 6 as well as exon 5 (four patients) to AF6 exon 2. In only three patients could the t(6;11) also be identified by cytogenetic and/or fluorescence in situ hybridization (FISH) analysis. The MLL-AF6-positive patients were monitored by RT-PCR for a period of 6-33 months. Complete haematological remission (CR) was achieved in all six cases, but was short in 5/6 patients (range 2.6-8.3 months). In these five patients, the MLL-AF6 transcripts were detected in every sample tested after induction and consolidation chemotherapy. One patient received autologous bone marrow transplantation (BMT) which also did not lead to PCR negativity. Intensive salvage therapy was unable to induce a second remission in the relapsed patients. One of the six MLL-AF6-positive patients achieved a molecular CR. He is still in CR at 33 months after diagnosis. Survival analysis indicates a poor prognosis in MLL-AF6-positive patients. The median event-free survival was 6.8 months, the median overall survival 15 months. Persistent PCR positivity was consistently associated with relapse. Thus, RT-PCR provides a valuable and sensitive tool for the identification of t(6;11)-positive AML and the monitoring of response to treatment in these patients. The results of RT-PCR may be useful to evaluate therapeutic procedures and to make treatment decisions, which will enable molecular remissions to be achieved and improve the clinical outcome in this group of patients.