The medical history of a three-year-old boy with acute ataxia due to the opsoclonus-myoclonusataxia syndrome (short: opsoclonus-myoclonus syndrome (OMS)) is presented. The further diagnostic investigation reveals an associated thoracic neuroblastoma. In children with acute ataxia, the diagnosis of OMS is rare, difficult and often unrecognized in the beginning. In half of the children with OMS, an associated neuroblastoma is present. Further investigation for an associated neuroblastoma is always indicated when the diagnosis of OMS is set.