The results of 701 determinations of antiepileptic drug plasma concentrations administered to 190 patients are described. It has been possible to reduce the number of prescribed drugs to 1.55 per patient, so that only 8.1% of subjects takes three or more drugs while 53% is on monotherapy. The use of the measurement of AEDs plasma concentrations resulted very useful: a) when Phenytoin (PHT) is prescribed; b) in epileptic children; c) when the patient takes two or more drugs; d) to evaluate the compliance. A significant increase (p less than 0.01) of the level/dose ratio of Phenobarbital (PB) when PHT is in, or over, the therapeutic range was observed, while at plasma concentrations of PHT below 10 micrograms/ml it does not influence the metabolism of PB.