Pharmacokinetics of vancomycin and dosage requirements were evaluated prospectively in 28 pediatric cancer patients 9 months to 13 years of age. The predictive performance of a two-compartment Bayesian forecasting program was also evaluated. A mean (±SD) daily dosage of 75 ± 22 mg/kg/day was necessary to attain a mean peak serum vancomycin concentration (SVC) of 23.1 ± 5.8 mg/liter and a mean trough SVC of 6.2 ± 2.3 mg/liter. Mean vancomycin clearance, volume of distribution and serum half-life were 0.153 ± 0.033 liter/hour/kg, 0.63 ± 0.08 liter/kg and 2.95 ± 0.48 hours. Final peak SVCs, which reflected the last dosage regimens received, were predicted with minimal bias (mean prediction error, −1.2 mg/liter) and accurate precision (root mean-squared prediction error, 2.0 mg/liter) whereas trough SVCs were predicted with even smaller bias (mean prediction error, −0.1 mg/liter) and greater precision (root mean-squared prediction error, 0.8 mg/liter). This study showed that pediatric cancer patients with normal renal function required vancomycin dosage regimens substantially greater than the standard 40 mg/kg/day to attain the desired SVCs.