Preoperative cytological diagnosis is very important for the treatment of mediastinal neuroblastoma. Recently, cytological smears of mediastinal tumor can be obtained by percutaneous needle biopsy. The purpose of this paper is to know whether subtype of mediastinal neuroblastomas can be diagnosed or not. Findings of needle biopsy smears were compared with that of tumor stamps and histological sections.Cytological characteristics of these tumors are followings:1. Mature tumor ganglion cells were characterized by abundant, dark bluish cytoplasma which has brown pigment granules. The nuclei were single or several and nucleoli were enlarged.2. Small immature tumor cells were pleomorphic and almost naked. A few rossete formation were found.A ganglioneuroma consisted of mature ganglion tumor cells and a ganglioneuroblastoma had both mature and immature tumor cells, and many transistional type of these cells were found. In neuroblastoma smears, only immature tumor cells were found.It was concluded that ganglioneuroma, ganglioneuroblastoma and neuroblastoma could be diagnosed by characteristical findings of tumor cells obtained by percutaneous needle biopsy.