Background: Phenytoin metabolism has been shown to vary in different ethnic groups. Therefore, the authors studied phenytoin pharmacokinetics in Indonesian adult patients with epilepsy. Objective: The purpose of this study was to determine and analyse the pharmacokinetic parameters of Michaelis Menten Vmax (maximum velocity) and Km (the Michaelis constant) of phenytoin in adult epileptic patients. Method: Twelve adult epileptic patients were chosen on the basis that they had two reliable steady-state phenytoin serum levels at two different daily doses and they required an additional adjustment of phenytoin dosage monotherapy. Result: The Vmax and Km values for adult epileptic patients respectively, ranged from 3.78 to 9.65 mg/kg/day and from 0.71 to 5.58 mg/L and there was no correlation with age and weight (p > 0.05). The Vmax and Km values have shown individual variability. Conclusion: The optimal adjustment of phenytoin dose individually in adult epileptic patients based on Vmax and Km parameters needs to be done to get the desired steady-state level according to the clinical response.