Response to treatment is the most important prognostic factor in children undergoing treatment for acute lymphoblastic leukemia. Factors that interfere with the pharmacokinetic determinants of anti-leukemic compounds may cause a decreased activity of one or more of the drugs included in the treatment protocol. Noncompliance with the prescribed treatment, individual pharmacokinetic differences, interference with oral drug absorption, and different guidelines for dose reduction related to toxicity are some of the factors responsible for a decreased or variable antileukemic activity which may influence treatment results.