We introduce a novel automated method for localization and quantitative comparison of delta activity in epilepsy patients. Comparison with healthy controls shows significantly increased slow waves in patients with and without recurring seizures. Significant correlation of delta and interictal epileptic activity could not be explained by the resection alone. Relation to spike localizations and resection volume imply involvement in seizure recurrence. The quantity of delta activity could be used as a diagnostic marker for recurrent seizures.