The prognostic impact of t(12;21)(p13;q22) [ETV6/RUNX1 fusion] in paediatric acute lymphoblastic leukaemia (ALL) has been extensively debated, particularly with regard to the frequency of late relapses and appropriate treatment regimens. We have retrospectively collected 679 ALLs with known ETV6/RUNX1 status, as ascertained by fluorescence in situ hybridization or reverse-transcription polymerase chain reaction, treated according to the Nordic Society of Paediatric Haematology and Oncology -ALL-1992 protocol. The assigned risk groups/treatment modalities for the 171 (25%) patients with t(12;21)-positive ALLs were 74 (43%) standard risk, 71 (42%) intermediate risk and 26 (15%) high risk. The 5- and 10-year event-free survival (EFS) of the 171 patients was 80% and 75% respectively, with no significant differences among the three risk groups. Most of the relapses occurred in boys and were late, with almost 50% of all relapses occurring > or = 5 years after diagnosis. Of all relapses after 6 years, 80% occurred in the t(12;21)-positive group. The overall survival was 94% at 5 years and 88% at 10 years; thus, the treatment of patients in second or later remission is usually successful. As yet, there is no reliable plateau in the EFS curve, a fact that raises the question as to when the prognostic ramifications of ALLs harbouring ETV6/RUNX1 should be evaluated.