Five children with subacute or acute onset of cerebellar ataxia and opsoclonus are described. Two had cerebrospinal fluid pleocytosis at the onset of ataxia and were initially thought to have acute parainfectious cerebellar ataxia of childhood. All were found to have tumors of neural crest origin (two neuroblastomas, three ganglioneuroblastomas). Tumors were small and only found by computed tomographic techniques. Urinary catecholamine metabolites were elevated in only two of the patients. Four of the five failed to improve neurologically with resection of the tumor. All four have had a steroid-sensitive chronic ataxic syndrome that worsens with acute nonspecific illnesses and has resulted in long-term deficits, particularly in speech and gross motor function. This is a metabolic encephalopathy associated with permanent residual neurologic deficits but without visible lesions on neuroimaging studies. We stress the frequency of cerebrospinal fluid pleocytosis in patients with tumor-associated opsoclonus and the clinical difficulty in separating tumor-associated cases from those due to other causes [corrected].