Multiple etiologies should be considered in the differential diagnosis of immunocompromised patients with non-central nervous system cancer and viral infections who develop mutism. Acute cerebellitis, caused by infections or by neurotoxicity resulting from chemotherapy; paraneoplastic cerebellar degeneration; atypical posterior reversible encephalopathy syndrome; and acute disseminated encephalomyelitis may all cause mutism in such patients. This condition warrants prompt recognition and may require treatment with immunotherapy, as it may be an immune-mediated process. We present 2 patients with leukemia and viral illness who developed cerebellar mutism in the setting of acute cerebellitis and responded to immunotherapy, suggesting that the condition involved a parainfectious immune-mediated response.