A light- and electron-microscopic review of 40 cases diagnosed at Children's Hospital of Pittsburgh as ganglioneuroblastoma, neuroblastoma, or small round cell tumor—probably neuroblastoma disclosed four cases that contained abundant glycogen. Two were unquestionable neuroblastomas by electron microscopy; one was primary in the adrenal gland, the other in the mediastinum. In the third case, a paraspinal tumor, the light-microscopic appearance was suggestive of neuroblastoma, but no catecholamine granules or neural processes were demonstrated in the material available for electron microscopy. The fourth case was an undifferentiated malignant tumor in the pectoralis muscle of a 12-year-old girl. By electron microscopy, neural processes were demonstrated and the tumor was classified as peripheral neuroblastoma. Of the remaining 36 cases, electron microscopy readily indicated a diagnosis of neuroblastoma or glangioneuroblastoma in 35 of them. In the other case, the tissue had been fixed in formalin and only a few catecholamine granules were found after an extensive search.