Correlation of abnormalities of interictal n-isopropyl-p-iodoamphetamine single-photon emission tomography with focus of seizure onset in complex partial seizure disorders. Single-photon emission tomography (SPECT) scanning with n-isopropyl-p-iodoamphetamine (IMP) was performed on 23 patients with complex partial seizures undergoing long-term video electroencephalographic (EEG) seizure monitoring. Twenty-one of the 23 patients had abnormalities on SPECT scanning consisting of areas of decreased activity reflecting diminished cerebral blood flow. In 15 of these 21 patients, there was good correlation between the site of the abnormality on SPECT scan and the site of origin of seizures monitored by EEG. Of the six remaining patients, four had multifocal SPECT abnormalities, with one of the abnormal areas corresponding with an ictal site. The two remaining patients had SPECT abnormalities and major ictal EEG foci in entirely different areas. In contrast to the high proportion of abnormal SPECT scans, only 10 of 23 focal abnormalities were discovered on magnetic resonance imaging (MRI) scans. Three patients who had seizures within 2 h of an initial scan were rescanned 4-5 h after injection. Focal areas of increased blood flow were noted on all three scans, although not always at the ictal site. The SPECT scan appears to be useful in interictal localization of seizure foci. Postictal scans may also be useful, although our numbers are too small to draw conclusions. 