To elucidate the evolution of crossed cerebellar diaschisis, cerebral oxygen metabolism was measured repeatedly by positron emission tomography (PET) in 35 consecutive patients with unilateral cerebral infarction within the territory of middle cerebral artery. The crossed cerebellar diaschisis was defined as significant when the laterality ratio of cerebral oxygen metabolism between the left and right cerebellar hemispheres exceeded the control range (mean +/- 2 standard deviations) as derived from 27 age-matched normal volunteers. Significant crossed cerebellar diaschisis was observed in 31 patients (89%) on the initial PET studies. Of these 31 patients, 23 with infarcts involving the frontal sensorimotor cortex persistently had crossed cerebellar diaschisis up to 5 years after onset, whereas the diaschisis disappeared in 8 patients with smaller infarcts mainly in the frontal or parietal lobe without recovery of oxygen metabolism in the infarcted areas. These present results suggest that crossed cerebellar diaschisis can exist persistently even in the late stage in those having a lesion involving the cortical pontine-cerebellar pathways.