Within the past 11 years, 11 patients with opsoclonus and myoclonus, with or without a history of neuroblastoma, have been admitted to Children's Memorial Hospital. Eight of the 11 children had an occult neuroblastoma. Eight children have had subsequent delayed development with motor incoordination and speech delay (7 with neuroblastoma, 1 without). Nine of 11 children initially were treated with ACTH, 1 child was treated with prednisone, and 1 was not treated. Nine of the 10 children who were treated had recurrences of symptoms during the gradual withdrawal or discontinuation of ACTH. Often the ACTH had to be restarted or increased, although several times the episodes were self-limited, not requiring treatment after ACTH was withdrawn. We found prednisone was ineffective in controlling opsoclonus-myoclonus regardless of etiology. The majority of children with opsoclonus-myoclonus, regardless of etiology, have developmental delay, more severe and at a higher rate than previously reported. When a neuroblastoma was present, tumor removal did not improve symptoms. Although limited in size, our study indicates patients with opsoclonus-myoclonus without an associated neuroblastoma have a better chance for normal neurologic development (2/3 versus 1/8).