We studied the electroencephalographic pattern at the time of focal seizure onset in 41 patients. Progressive rhythmic EEG activity was characteristic of spontaneous focal seizures in man. Focal hypersynchrony was present in 49% of cases and served as an accurate localizing indication of cortical irritability. Interictal sharp or spike discharges were absent in 34% of cases. When present, spike and sharp discharges characteristically attentuate at the onset of rhythmic ictal activity. Recognition of the pattern of rhythmic ictal transformation is often essential for the electroencephalographic diagnosis of focal seizures.