SUMMARYAn 18-month-old girl presented with incoordination and involuntary movements of acute onset. No viral etiology was proved. Cerebellar atrophy was suggested by pneumoencephalography. Opsoclonus set in later. Symptoms could be suppressed by large doses of corticotropin. Subsequently, a neural crest tumor (ganglioneuroblastoma) was suggested by chemical studies and removed surgically. After an initial three-month asymptomatic period, polymyoclonia has returned in spite of normal catecholamine secretion, but symptoms disappear on smaller doses of ACTH than previously.