The study presents ultrastructural findings in 89 neuroblastic tumors. All of them including 21 undifferentiated neuroblastomas (NB) were characterized by neural cell processes which contai- ned neurofilaments, mitochondria and neurosecretory granules. The granules were generally sparse and their number and distribution was a feature independent on other signs of differentia- tion of the tumor cells. We found degenerative changes in the tumor cell bodies and their neuro- nal processes to be a prominent phenomenon in neuroblastic tumors. Although they occurred both in patients who were operated primarily and in those who received a preoperative chemo- therapy they were most conspicuous in the latter group (28 of 80 patients). The degenerative changes included swelling and dilatations of the neuronal processes, accumulation of heteropha- gosomes, myelinated bodies and cytoskeletal filaments. Some ganglion cells had dilated endoplas- mic reticulum, and contained lipofuscin. In one ganglioneuroblastoma we detected dense granulated bodies resembling neuromelanin. The diagnosis of undifferentiated NB was estab- lished on the basis of ultrastructural investigations in 13 out of 21 undifferentiated NB; in the remaining 8 cases the electron microscopy was an additional confirmatory tool together with the results of immunohistochemical reactions. The electron microscope is still a valuable instrument for diagnosis of neuroblastic tumors in addition to newer diagnostic approaches such as immuno- histochemical and molecular investigations.

        Key words: neuroblastoma - ganglioneuroblastoma - ganglioneuroma - ultrastructure - degenerati- ve changes