A list (n = 423) of phenytoin steady-state concentrations from children taking phenytoin alone or in combination with other drugs (n = 308) was obtained from our therapeutic drug monitoring database (1984-1990). Only 43% of concentrations were within the commonly accepted "therapeutic" or "reference" range (10-20 mg/L). Age-dose-concentration relationships showed that while occasional concentrations were above 20 mg/L, many patients receiving commonly recommended pediatric phenytoin doses (4-8 mg/kg/day) achieved concentrations below the reference range, especially children under 3 years of age. Michaelis-Menten pharmacokinetic parameters, maximal elimination rate (Vmax) and Michaelis constant (Km), were evaluated for patients with at least two steady-state concentrations measured for two or more different daily doses. Comedication with other drugs had no effect on either Vmax or Km for phenytoin. Vmax appeared to decrease significantly with age, but there was no age relationship for Km.