A method to assess changes in cerebral perfusion following carotid endarterectomy was developed using late (2 hr)/early (20 min) single-photon emission computed tomography (SPECT)/[123I]iodoamphetamine count ratios. Using a ratio greater than 1.0 to indicate redistribution and reversible ischemia, pre- and postoperative studies were compared for 20 patients. Regional polar plots based on 30 degrees angular sectors showed improvement greater than 2 standard deviations (s.d.s) ipsilateral to surgery in 15/19 (79%) and contralateral to the side of surgery in 8/19 (42%) patients with significant hemodynamic lesions. Using a hemispheric perfusion index (mean of four 30 degrees sectors) ipsilateral perfusion improved in 11/19 (58%) with bilateral improvement in 6/19 (32%). Visual interpretation was similar to the regional analysis with 14/19 (74%) improving on the operative side; however, it was less sensitive for contralateral changes, 4/19 (21%). We conclude that quantitation of redistribution can provide an objective index of improved perfusion and is especially important to detect contralateral changes.