BackgroundTherapeutic drug monitoring (TDM) of asparaginase (ASNase), a fundamental element of acute lymphoblastic leukemia treatment, was integrated in the ALL‐BFM 2000 protocol on a voluntary basis.MethodsOver a 5‐year period, 127 patients (1,355 samples) were monitored for asparaginase activity in a single‐center setting. We report monitoring data from throughout the ASNase containing treatment elements. Additional information obtained on risk stratification, minimal residual disease (MRD), steroid randomization and relapse is discussed in relation to ASNase activity.ResultsAt completion of the induction phase 93% (115/124) of patients showed sufficient ASNase activity (5,000 U/m2 Escherichia coli ASNase), 77 of 86 (90%) monitored patients finished the first re‐intensification element without requiring Erwinia ASNase. MRD, risk stratification and steroid randomization were not associated with significant differences in ASNase activity. Of patients who relapsed, only 25% (3/12) were able to maintain sufficient ASNase activity after E. coli ASNase.ConclusionThis single‐center data set gives a true and unbiased insight into clinical reality of ASNase therapy. It shows no significant relationship between MRD positivity or risk stratification and ASNase treatment intensity. Overall, within the ALL‐BFM 2000 trial, 90% of patients completed first re‐intensification without requiring third‐line Erwinia ASNase. Pediatr Blood Cancer 2011; 57: 378–384. © 2011 Wiley‐Liss, Inc.