Positron emission tomographic studies of 16 patients with cerebral ischemia and brain tumor showed asymmetries in cerebellar metabolism not encountered in 14 normal control subjects. An asymmetry was present in 62.5% of cases. The lower metabolic rate occurred in the cerebellar hemisphere contralateral to the cerebral lesion (p less than 0.001; sign test). In all cases computed tomography showed the supratentorial lesion to be unilateral and the posterior fossa contents to be unaffected. The presence of depressed cerebellar metabolism was highly associated with involvement of the contralateral parietal lobe (p less than 0.02; phi coefficient). The presence of a cerebellar abnormality was not related to the presence of any particular sign. Serial studies showed normalization of cerebellar metabolism over time. It is likely that this effect is a result of interruption of the functional interconnections between the cerebrum and the cerebellum.