Concentrations of methotrexate have been determined in the serum and cerebrospinal fluid after 406 infusions of methotrexate to 58 children with acute lymphoblastic leukemia. The dose of methotrexate varied between 500 mg/m2 and 33,600 mg/m2. Pharmacokinetic analysis of the data has been carried out. The effect of dose, age, and number of treatments on steady-state concentration, serum half-life, cerebrospinal fluid (CSF) serum distribution ratio, volume of distribution, systemic clearance of methotrexate was examined. The elevation of dose resulted in a nonproportional increase of the steady-state concentrations both in serum and CSF. The great inpatient and interpatient variations of steady-state concentrations caused only statistically not significant differences in the parameters of different subgroups of dosages. Correlation was found between concentrations of methotrexate in serum and CSF. One to 4 years old children were found to have lower steady-state concentrations of methotrexate in the serum and CSF, greater volume of distribution and faster clearance of the drug. Dose-dependency and age-dependency of methotrexate pharmacokinetics has been concluded.