Abundance of beta activity in epileptic patients under clonazepam (Rivotril) treatment shows a significant correlation with the suppression of epileptic seizures and/or of epileptic EEG activity such as spikes and spike/wave complexes. In a longitudinal two-year study in 21 children with severe epilepsy treated with clonazepam, relative spectral peak power (RSPP) of beta activity was correlated with daily dose and serum concentration as well as with its effect on seizures and on epileptic EEG activity. Eight cases showed significant correlation of beta RSPP to daily dose, but only 4 to serum concentration. A significant (inverse) correlation was found between beta RSPP and the relative occurrence of epileptic EEG activity. Most illustrative were the individual follow-ups, with beta RSPP as an interesting additional parameter regarding the course of the epileptic condition. Quantitative measurement of beta activity by means of spectral analysis during long-term antiepileptic treatment promises important additional information on specific aspects in the individual case.