Interictal epileptiform activity (IIEA) on EEG may be useful in the diagnosis and characterization of epilepsy. A single scalp EEG may fail to detect this abnormality, which is then discovered on repeat recordings. Since many people with epilepsy have persistently normal EEGs, a practical question arises concerning the yield of multiple EEGs. To determine the probability of finding IIEA with serial EEGs (the operational curve), we reviewed data from 1,201 EEGs on 429 adult patients, most with definite epilepsy presenting in adulthood. The diagnosis was made either clinically or with the support of the EEG. In 50% of our patients with IIEA, the abnormality is present on the first record, in 84% by the third EEG, and in 92% by the fourth. There is relatively little yield to serial EEGs beyond this point.