A multicenter prospective study was conducted in 114 children with acute lymphoblastic leukemia receiving 4 intravenous methotrexate (MTX) courses (5 g/m2 as a 24 hour-infusion) to determine the diffusion of MTX in cerebrospinal fluid (CSF) and to correlate the drug levels to central nervous system (CNS) relapse occurrence. Serum and CSF levels were measured at the end of 446 MTX courses. A significant correlation was found between CSF and serum MTX concentration. CSF MTX level was greater than 1 mumol/l in 66% of the courses. Twelve patients (11%) failed to achieve this potentially cytotoxic drug level at the end of the 4 consecutive MTX courses: only one CNS relapse was observed and the mean age of these children was lower than that of the others. To date 9 (7.8%) children had CNS relapse and no difference was observed in terms of CSF MTX levels when compared to data of children free of CNS relapse. With a median follow up of 32 months, pharmacokinetic data during high-dose MTX therapy do not seem to be an exclusive predictive factor of CNS relapse.