23 patients with osteogenic sarcoma were observed during 142 6-hour high-dose infusions of methotrexate (MTX, 3,000--8,200 mg/m2). Calcium leukovorin was given by intravenous injection at 3-hour intervals beginning 2 h after the completion of each MTX infusion with extension of the intervals to 6 h following the first day of rescue. All patients also received continuous intravenous infusions of alkalinized fluids for the entire duration of leukovorin rescue. No larger doses of leukovorin were given to any patient. Three of the 142 MTX infusions resulted in mild cytotoxic side effects. Plasma MTX clearance ranged from 90 to 600 ml/min among the 62 infusions where plasma clearance could be accurately calculated. The 3 patients with mild toxicity had low drug clearance, but others with similar low MTX clearance experienced no apparent toxic effects beyond the expected transient nausea.