The pharmacokinetics of 222 infusions of high-dose methotrexate (MTX) with leucovorin rescue were studied in 22 adults with osteosarcoma. To reduce the variability of plasma concentration, we individualized dose regimens using a Bayesian method to reach a concentration of 10(-3) M MTX at the end of an 8-h infusion. The mean concentration observed at the end of the infusion was 1016 +/- 143 mumol/l. The mean dose delivered was 13.2 +/- 2 g/m2. The clearance was 49.1 +/- 11.7 ml min-1 m-2. The decay of the plasma concentration of MTX after completion of the infusion followed a two-compartment model with a t1/2 alpha of 2.66 +/- 0.82 h and a t1/2 beta of 15.69 +/- 8.63 h. The volume of distribution was 0.32 +/- 0.08 l/kg. As compared with previously published data, the interindividual and intraindividual variations in the concentration at the end of the infusion were reduced, with values of 14% and 5.9%-21%, respectively, being obtained. Severe toxicities were avoided, and there were only 3 hematologic and 8 digestive grade 3 side effects and no grade 4 complication. The t1/2 alpha and the MTX plasma concentrations at 23 and 47 h were correlated with renal toxicity (P < 0.001). However, no correlation was found between the pharmacokinetic parameters and other signs of toxicity. There was no significant difference in pharmacokinetics between the toxic and nontoxic groups. In the same manner, the parameters of the group of patients sensitive to MTX were not statistically significant different from those of the group of nonsensitive patients.