9 HDMTX infusions with 30 to 55 g MTX and leucovorin rescue were given to 3 patients with multifocal osteosarcoma. Resistance to standard MTX infusions with up to 12 g/m2 body surface in 4 hours indicated the administration of these higher doses (27 g/m2 body surface on average) over a shorter period of time (160 min on average). MTX input was controlled by the increase in plasma MTX levels which were determined by EMIT enzyme assay within a few minutes of sampling. The plasma MTX levels were found to be up to 10 times (average 4 times) higher than those achieved by the standard infusion. All infusions proved to be relatively well tolerated in all 3 cases, although, thrice after 36 hours an increased leucovorin rescue had to be made for 12 hours, because of delayed renal methotrexate excretion; the usual leucovorin rescue (15 mg every 6 hours) was given for an average of five days. No therapeutic effects were seen either clinically, radiologically, or histologically. The resistance to HDMTX in these three patients could not be broken by HDMTX PLRD infusions.