The present pharmacokinetic (PK)-pharmacodynamic (PD) study will explore the toxicity and antileukemic response during the initial 3 months of individualised therapy of children and young adults with acute lymphoblastic leukemia (ALL). The investigators will on an individual toxicity-titrated basis attempt to increase the dose intensity of the 6-mercaptopurine used in the two-months post-remission treatment phase of lower risk childhood ALL. This will be performed together with continuous PEG-ASP (every 2nd week) and interspersed HD-MTX (5 g/m^2) every 3rd week. Thus, the trial will also test the feasibility of this particular drug combination.