Ganglioneuroma is a rare, differentiated, benign and slow-growing tumor that commonly arises from sympathetic ganglion cells. Most of them are asymptomatic and found incidentally. We here report a quite rare case of silent huge ganglioneuroma growing in both posterior mediastinum and retroperitoneum occurring in a 3.5-year-old girl. The patient was relatively well before and incidentally found to have a huge chest mass by chest X-ray film at an episode of respiratory tract infection. Computed tomography showed a huge tumor extending from bilateral posterior mediastinum to the level of the adrenal gland in the retroperitoneum. Initially, neuroblastoma was highly suspected and 24-hour urine vanillyl mandelic acid was slightly elevated. Cytology by bone marrow aspiration revealed no tumor nests or clumps. Biopsy and pathology proved it as ganglioneuroma (GN). Due to too extensive involvement of the tumor and compression of the vital vessels, surgical removal became difficult. The family of the patient refused surgery due to there being no significant symptoms. Because of the potential for growth of unresectable GN and because the component of neuroblasts could not be completely excluded, the patient was still in dangerous status. The only thing we can do is to keep the family alert and continue regular follow-up.