were compared, white cell and absolute neutrophil count (ANC) nadir, lymphocyte count nadir, thrombocyte count nadir, and hemoglobin nadir after HDM decreased significantly with increasing doses of oral 6MP. Three percent of the HDM courses given without oral 6MP (SR consolidation) were followed by an ANC nadir Ͻ0.5 × 10 9 /l compared to 50% of the HDM courses given during SR/IR maintenance therapy. Similarly, only 13% of the HDM courses given as SR-ALL consolidation induced a thrombocyte count nadir Ͻ100 × 10 9 /l compared to 58% of the HDM courses given during maintenance therapy. The best-fit model to predict the ANC nadir following HDM during maintenance therapy included the dose of 6MP prior to HDM ( = −0.017, P = 0.001), the average ANC level during maintenance therapy ( = 0.82, P = 0.004), and E-6TGN ( = −0.0029, P = 0.02). The best-fit model to predict the thrombocyte nadir following HDM during maintenance therapy included only mPLATE ( = 0.0057, P = 0.046). In conclusion, the study indicates that reductions of the dose of concurrently given oral 6MP could be one way of reducing the risk of significant myelotoxicity following HDM during maintenance therapy of childhood ALL. Leukemia (2001) 15, 74-79.