Asparagine levels in cerebrospinal fluid and serum asparaginase activities were monitored in children with acute lymphoblastic leukemia treated with PEG-asparaginase. The drug was given intravenously at 2,500 IU/m2 on days 12 and 26. Serum and cerebrospinal fluid samples obtained on days 33 and 45 were centrally analyzed. Since physiological levels of asparagine in cerebrospinal fluid of children and adolescents are 4-10 μmol/L, in this study asparagine depletion was considered complete when ≤0.2 μmol/L, i.e. below the lower limit of quantitation of the assay used. Over 24 months 736 patients (AIEOP n=245, BFM n=491) and 903 cerebrospinal fluid samples (686 day 33 and 217 day 45) were available for analysis. Data were analyzed separately for the AIEOP and BFM cohorts and yielded superimposable results. Independently of serum asparaginase activity levels, cerebrospinal fluid asparagine levels were significantly reduced during the investigated study phase but only 28% of analyzed samples showed a complete asparagine depletion while relevant levels ≥1 μmol/L were still detectable in around 23% of them. Complete cerebrospinal fluid asparagine depletion was found in around 5-6% and 33-37% of samples at serum asparaginase activity levels <100 and ≥1,500 IU/L, respectively. In this study cerebrospinal fluid asparagine levels were reduced during PEG-Asparaginase treatment, but complete depletion was only observed in a minority of patients. No clear threshold of serum PEG-Asparaginase activity levels resulting in a complete cerebrospinal fluid asparagine depletion was identified. The results consistently found in the two independent data sets strengthen the observations of this study.