An unusual tumor of the cystic duct in a 28-year-old woman is described. The patient presented with a painful distended gallbladder due to a small tumor occluding the cystic duct. Microscopically the tumor cells showed a nesting pattern suggestive of endocrine differentiation, but contained numerous lipid vacuoles and were argentaffin and argyrophil negative. Ultrastructurally, there were relatively few dense granules measuring 135 to 475 nm. Immunoperoxidase staining showed that the tumor cells contained somatostatin but did not contain immunoreactive ACTH, gastrin, calcitonin, glucagon, insulin, parathyroid hormone, or carcinoembryonic antigen. To the authors' knowledge, this is the first reported somatostatinoma occurring in the extrahepatic biliary tract.