A study of gangliocytic paragangliomas (GPs) of the gastrointestinal tract from 51 patients showed characteristic microscopic features: epithelioid cells with an endocrine growth pattern, spindle cells, and ganglion cells. Forty-nine tumors were located in the duodenum, 1 in the jejunum, and 1 in the pylorus. Twenty-one patients were female, 28 male, and for two the sex was unknown. The average age at presentation was 54 years (range, 23-83). No patient had a recurrence. No neuroendocrine syndrome was found in any patient or patient's family. Immunohistochemical stains in 33 cases yielded the following (proportion positive): S-100 protein 94%, synaptophysin 94%, neuron-specific enolase 94%, pancreatic polypeptide 88%, somatostatin 75%, chromogranin 72%, neurofilament protein 64%, keratin 52%, leu-enkephalin 48%, serotonin (one case), and gastrin (one case). Antisera usually stained one or two of the three major cell types. Pancreatic-type tissue was identified in or near 28 tumors, including the pyloric and jejunal lesions and two in the distal duodenum. The authors conclude that GP is benign; is not associated with endocrine syndromes; contains autonomic, neural, and endocrine cell types; and is related to pancreatic development.