N-Isopropyl 123I P-iodoamphetamine (IMP) is a radiopharmaceutical that is lipid-soluble, penetrates the normal blood-brain barrier, and has a high first-pass extraction efficiency in brain. Initial IMP distribution is proportional to local cerebral blood flow and is stable for at least 1 hr after administration. When combined with single-photon emission computed tomography (SPECT), regional activity can be quantified. Eighty-three studies were performed to aid in understanding the clinical utility of this radiotracer. In control subjects, brain activity followed expected regional perfusion patterns. Physiologic maneuvers produced altered perfusion patterns corresponding to those demonstrated by positron tomography. Perfusion abnormalities in stroke patients were visualized earlier than structural changes detected by transmission computed tomography (CT), and the area of altered IMP activity generally exceeded that suggested by standard CT examination. Seizure foci were identified by local abnormalities of cerebral blood flow. Primary brain tumors failed to demonstrate IMP activity even when shown by angiography to be highly vascular.