Four plexosarcomas (gastrointestinal autonomic nerve tumors) characterized by light microscopic, immunocytochemical, and ultrastructural examination, including immunoelectron microscopy in one case, are described. The four neoplasms occurred in the small intestine (duodenum, two; jejunum, one; and ileum, one) and they had an aggressive course with either local or distant metastases. The light-microscopic patterns varied from epithelioid and organoid to spindle cells, mimicking endocrine and sarcomatous neoplasms. Ultrastructurally, these tumors exhibited interdigitating cytoplasmic processes that contained scattered aggregates of membrane-bound granules varying in size from 100 to 300 nm intermixed with empty vesicles and numerous diffusely distributed intermediate filaments. Basal lamina covering cell surfaces, attachment plaques, and myofilaments, as expected in smooth-muscle tumors, were not identified, and diffusely distributed membrane-bound granules, as seen in paragangliomas and carcinoid tumors, were also absent. By immunocytochemistry, the tumors were intensely positive for vimentin and neuron-specific enolase and focally positive for neurofilaments and synaptophysin. In addition, three tumors were S1000 protein positive and one stained for vasoactive intestinal peptide. Similar positive immunocytochemical reactions were identified in normal enteric plexus. It is essential to recognize plexosarcomas, which are invariably accompanied by aggressive clinical behavior, in spite of a seemingly benign, mitotically inactive light-microscopic appearance in most instances. Ultrastructural examination can readily separate plexosarcomas from paragangliomas and other sarcomatous and endocrine neoplasms.