The study comprised 150 epileptic patients treated at the Institute of Neurology of the Clinical Centre Novi Sad. The optimal therapy with least adverse effects and seizures was achieved in patients in whom measurement of serum concentration of antiepileptic drugs was performed. Patients were divided into five groups with respect to the therapy they received: I--carbamazepane; II--valproic acid; III--polytherapy with phenobarbitone and diphenylhydantoin; IV--phenobarbitone and valproic acid; and V--phenobarbitone, valproic acid and carbamazepine. No adverse effects were recorded in over 60% of patients on monotherapy, 35% of patients who received two anticonvulsants, and 30% of patients who received three anticonvulsants. Significant correlation between drug dosage and blood drug concentration (r > 0.5) was found in polytherapy with phenobarbitone, carbamazepine and valproic acid (r = 0.66); and phenobarbitone and diphenylhydantoin (r = 0.53).