Ganglioneruroblastoma and neuroblastoma are among commonest types of childhood malignancy and a number of unique paraneoplastic syndromes have associated with both localized and disseminated neuroblastoma. The coincidence of neuroblastoma and myoclonic encephalopathy or other paraneoplastic syndromes occurs relatively rare, and therefore, failure to recognize this association could result in delays in both diagnosis and treatment, and the result could prove to be unfortunately fatal. The mechanism which underlies the remote damaging effect of neural crest tumor, especially neuroblastoma, on the nervous system resulting in myoclonic encephalopathy is by no means clear. In addition the nature and the extent of the pathologic lesion are inconsistent. We experienced a case of myoclonic encephalopathy associated with an occult mediastinal ganglioneuroblastoma in a 22-month-old girl who was hospitalized for inability to walk without support and tilting of the head to the left side. She became increasingly ataxic, and during the hospitalization myoclonic jerks of upper extremities and head along with chaotic, rapidly flickering, multidirectional spontaneous eye movements, were noted. Laboratory data included normal complete blood count, urinalysis, BUN and creatinine, electrolytes and bone marrow. Chest X-ray and chest CT revealed a relatively well marginated right posterior mediastinal mass. In a 24 hours urine excretion test, VMA and catecholamines were increased. Over the next 2 weeks, a surgical exploration revealed a right posterior mediastinal mass. Microscopically the mass proved to be a ganglioneuroblastoma, extending to right innominate artery and right axillary lymph nodes. Within 2 weeks after the surgery, radiotherapy (2,400 rads) and chemotherapy (CTX, DTIC, VCR) were started, but corticosteroid was not used. She has been free of tumor and abnormal neurological systemic symptoms and signs for 1 1/2 year since the completion of chemotherapy. In the 3 1/2 years follow-up period, her neurologic symptoms has completely resolved by the completion of 2 years chemotherapy. We report a case of mycoclonic encephalopathy associated with hidden ganglioneuroblastoma in 22-month-old girl.