The effect of carbamazepine (CBZ) on EEG background activity and on interictal epileptiform abnormalities (IEA) was studied in 15 patients with focal epilepsy who started antiepileptic drug treatment (AED) for the first time. A computerized EEG study, performed before and two months after starting therapy, utilized spectral analysis and automatic recognition of IEA. The occurrence of seizures was considered in order to clarify the relationship between IEA and the seizures themselves. Statistical evaluation of EEG spectral values was calculated through analysis of variance (ANOVA), while for IEA the non parametric Wilcoxon test was utilized. In 13 patients who remained seizure-free during the two month-period of observation, IEA was unchanged or decreased. In one of the two patients who continued to have seizures, IEA significantly increased. During therapy, the background activity presented significant increase of slow activity in approximately half the patients. This increase was more evident in those patients showing active epileptic foci before treatment, rather than in those with rare or no spikes. The changes in IEA seem to be related to the occurrence of seizures rather than to a direct action of CBZ on spikes.