Described in this paper are histological, electron-microscopic, and immunocytochemical findings recorded from a duodenal gangliocytic paraganglioma in a 21-year old man. The sessile polypoid tumor consisted of epithelial cells, neuroid spindle cells, and gangliocytic elements. Neurosecretory granules were detected by electron-microscopy in the epithelial cells. The neuroid spindle cells exhibited the ultrastructural feature of Schwann cells. The tumor was immunocytochemically characterized by the S-100-reactivity of the spindle cells and numerous PP-reactive epithelial cells. Gangliocytic paragangliomas of the duodenum are of benign behaviour and should be removed by local excision.