The specific EEG manifestations of epilepsy, seizures, and interictal spikes and sharp waves occur at unpredictable times and at variable frequencies. To obtain an adequate diagnosis, it is often necessary to record the EEG for several hours or several days. A computer system was developed to perform data reduction and quantification by continuously monitoring seizures and automatically recognizing interictal spikes and sharp waves. The past 2 min of EEG are kept on the computer disk at every instant. When an epileptic seizure occurs or when a spike is detected, a sample of EEG, including a section preceding the event itself, is written on the EEG polygraph and on magnetic tape. A continuous recording is thus replaced by samples of varying lengths, containing only the important aspects of the EEG, reducing considerably the original data. After the monitoring session, the spatial and temporal distributions of the interictal activity are presented in a quantified form on the terminal. The seizures are recorded on digital tape and are available for several types of processing. The patient is also monitored by a video system; EEG and video are synchronized by a time-of-day clock to allow electroclinical correlations.