The Michaelis-Menten pharmacokinetic parameters Vmaxand Km were calculated for 135 epileptic paediatric patients receiving phenytoin as their only anticonvulsant therapy. Mean Vmaxand Km values were 13.95 mg/kg/day and 6.59 &mgr;g/ml for 0.5 to 3-year-old patients, 10.93 mg/kg/day and 6.82 &mgr;g/ml for the 4 to 6 year age group, 10.05 mg/kg/day and 6.51 &mgr;g/ml for the 7 to 9-year-olds, and 8.25 mg/kg/day and 5.69 &mgr;g/ml for the 10 to 16 year group. Using analysis of variance, the Vmaxvalues were significantly different (p < 0.01) but the Km values were not. Linear regression analysis of Vmaxversusage revealed a significant decline in Vmaxwith age (r = -0.554; p < 0.001). A plot of Kmversusage showed a poor correlation (r = -0.170) and a large amount of variability.Based on this data, the youngest age group would require on average 62% more phenytoin/kg/day than the oldest age group in order to maintain a steady-state phenytoin concentration of 15 &mgr;g/ml. Because of these age-related pharmacokinetic differences, phenytoin dosages may require adjustment as paediatric patients become older.