Ictal wave form characteristics--frequency, spatial distribution, and duration--were analyzed for 140 complex partial seizures recorded from epidural strip electrodes implanted in 28 patients. None had abnormalities on imaging studies. All had bilateral electrode placements, unilateral seizure onsets, temporal lobectomies, and were followed for a mean of 33 months postoperatively. Sixteen patients (57%) became free of complex partial seizures: 12 had reductions in seizure frequency of at least 50% but were not seizure-free. The only predictor of the seizure-free state was the presence of low voltage fast activity (LVF), in the alpha or beta ranges, localized to one gyrus. This phenomenon occurred in 14/16 seizure-free patients, 2/12 of others (P < 0.001). As seizures progressed, LVF typically increased in amplitude, propagated, and slowed into the theta range. Wave forms were classified into 8 categories based upon their frequency and morphology. Stepwise discriminant analysis of these wave forms, with consideration of whether they were localized or regional, revealed that both frequency and localization were critical for the post-surgical prognosis. The mere presence of a localized seizure onset was unreliable unless the wave form was taken into account. Well-localized rhythmic activity over 8 Hz at seizure onset from epidural subtemporal electrodes predicts surgical success. Slower rhythms imply greater separation in space and time from seizure onset.