Frequency analysis (spectral analysis) of both EEG and evoked potentials were studied prospectively in 52 patients with complex partial seizures to see if such tests could help localize the epileptic focus. Results were examined in line, bar and topographic head display formats. Asymmetries of fast and slow frequency components were identified in 26/52 patients. The existence and localization of such frequency alterations agreed with the overall results of other tests including the routine EEG, ictal EEG records, thiopental-activated EEG, CT or MRI, positron computed tomography (PET), and neuropsychometric testing. Changes were present in all 5 temporal lobe epilepsy cases that had a focal CT or MRI change, and in a majority (17/33) of cases in which the baseline EEG showed no focal slowing. These changes agreed with the preponderance of evidence in lateralizing the epileptic focus. Overall, EEG and evoked potential frequency analysis and topographic mapping do appear to offer data which are useful and complementary to other available test results, especially when a suspected lesion does not already show up as a defect on the CT scan or as a well-defined EEG slow focus. Considerable care needs to be taken to identify artifacts and normal EEG variants when interpreting these tests.