Occlusion or severe stenosis, with a reduction in the diameter of more than 70% of the extracranial arteries may lead to hpoperfusion of the brain with an increased risk of cerebral infarction. The aim of this study was to investigate whether endarterectomy of stenosed internal carotid arteries leads to alternations in cerebral metabolism in regions in which no infarcts were visible with magnetic resonance imaging (MRI). We studied 10 healthy control subjects and 20 patients with transient or nondisabling cerebral ischemia with MRI and 1H magnetic resonance spectroscopic imaging. All patients underwent carotid endarterectomy. Patients were examined 1 week before and 3-6 months after carotid endarterectomy. The N-acetyl aspartate (NAA)/choline ratio in the symptomatic hemisphere before endarterectomy (2.29 +/- 0.42) was significantly (p < 0.001) lower than for control subjects (3.18 +/- 0.32). In five of the patients lactate was detected preoperatively in regions that were not infarcted. The NAA/choline ratio in the symptomatic hemisphere of these five patients did not increase significantly after endarterectomy (1.99 +/- 0.22 vs. 2.23 +/- 0.48). The NAA/choline ratio in patients without lactate preoperatively increased significantly (p < 0.01) after endarterectomy to a normal level (from 2.39 +/- 0.42 to 2.92 +/- 0.52). These results indicate that the presence of cerebral lactate may predict whether the NAA/choline ratio increases after carotid endarterectomy.