This paper evaluates the influence of pharmacokinetics monitoring of HDMTX in the treatment of localized operable previously untreated high grade osteosarcoma. 44 patients (group 1) received a T10 protocol with dose adapted only to age. 27 other patients (group 2) had a pharmacokinetics monitored dose adaptation of MTX. The pharmacokinetics monitoring leads to higher dosage, higher area under the concentration/time curve and permits higher toxicity to be avoided. The higher dose intensity of MTX gave higher histologic response rate (66% compared to 45%) and higher 5 year disease free survival (92% compare to 76%). HDMTX treatment of osteosarcoma should be dose adapted to indivi-dual pharmacokinetics.