Newly diagnosed children with ALL (n=32) were treated on a protocol incorporating minimal residual disease (MRD)-based treatment decisions. MRD was monitored at 4 time points by semi-quantitative PCR detection of antigen receptor gene rearrangement, flow cytometry, quantitative RT-PCR detection of chimeric gene transcripts and overexpressed WT1 mRNA. Four patients positive for MRD at week 5 were treated with an intensified regimen. Median follow-up was 5.0 years (range 3.8-6.6 years) with a 4-year event-free survival rate of 93.8+/-4.3%. This MRD-based treatment strategy seems to be highly successful and may improve the outcomes of children with ALL. A large study is warranted.