The authors review the mean steps for the treatment of osteogenic osteosarcoma from the 1970's: 1), demonstration of the effectiveness of HDMTX and possibility of weekly administration, dose-response effect, interest of other drugs (BCD, ADR, CDDP, IFX); 2), use of the primary as chemosensibility witness; 3), extent of conservative surgery. In order to optimize the good results obtained by Rosen (more than 80% DFS at 5y) the authors studied the HDMTX pharmacokinetics, the value of the seric peak at the end of infusion as an effective test and individualized the HDMTX treatment in each patient following his own pharmacokinetics. This individual approach allows us to obtain more than 90% actuarial event-free survival at 4 years in patients treated by conservative surgery.