Crossed cerebellar diaschisis is a condition in which cerebellar hypometabolism is ascribed to functional disconnection of the contralateral hemisphere from the cerebral cortex. Interruption of the cerebropontine-cerebellar pathway is thought to be the most likely mechanism of this remote transneuronal metabolic depression. This phenomenon can be diagnosed by positron emission tomography, as there is hypometabolism in the affected cerebellar hemisphere. The morphologic features of the affected cerebellar hemisphere in patients with the diagnosis of crossed cerebellar diaschisis have not previously been studied by MR imaging. We retrospectively reviewed 26 patients in whom the diagnosis of crossed cerebellar diaschisis was suggested by positron emission tomography; these patients also had MR studies. In all 26 patients, supratentorial diseases were documented by MR imaging. Twenty-four of the 26 patients had a pathologic diagnosis. Detailed clinical history was also obtained for all patients. MR findings of morphologic change in the affected cerebellum were correlated with the patient's clinical symptoms and supratentorial disease. Of the 26 patients, eight had cerebellar atrophy; the remaining 18 patients showed no MR abnormality of the affected cerebellum. In the 18 in whom no atrophy was seen on MR imaging, the majority of the supratentorial lesions were tumors. The eight patients in whom cerebellar atrophy was demonstrated usually had significant contralateral supratentorial hemispheric atrophy. The patients with atrophy were also generally younger (average age, 14 years) than the 18 patients without cerebellar atrophy (average age, 42 years). Our experience shows that a significant number of patients with crossed cerebellar diaschisis have morphologic changes of cerebellar atrophy shown by MR imaging. These patients usually have significant contralateral supratentorial hemispheric atrophy.