The aim of the study was to investigate the conditions under which contralateral hemispheric hypometabolism (CHH) occurs among patients with chronic, stable middle cerebral artery (MCA) infarcts. Twenty-eight patients with chronic MCA infarction and 22 healthy volunteers were studied using positron emission tomography and the oxygen-15 (15O) steady-state technique. In 43% of our patients decreased regional cerebral metabolic rate of O2 (rCMRO2) in the contralateral mirror region and in the total contralateral MCA territory was observed. In patients with severe bilateral carotid disease regional cerebral blood flow and rCMRO2 were more diminished than in those without, suggesting unsuspected ischaemic changes in the contralateral cerebral hemisphere. However the existence of transhemispheric diaschisis (THD) was still suggested as the rCMRO2 values were also significantly lower in the contralateral hemisphere of patients with isolated unilateral vascular lesions in comparison to healthy volunteers. The presence of CHH and THD was related to more severe ischaemic changes in the homolateral cerebral hemisphere with predominant infarction in the cortical end zones of the MCA.