An extremely rare case of psammomatous calcification in a nonfunctioning duodenal carcinoid tumor was studied by light and electron microscopy. The psammoma bodies were found most frequently within the lumina of the neoplastic glands. The ultrastructural features of the psammoma bodies were the marginal needle-shaped crystals and a coarse granular matrix in which a few organelles were embedded. The psammoma bodies appeared to be occasionally intracytoplasmic in position, showing no distinct limiting cytoplasmic membrane. Electron probe x-ray microanalysis revealed that the needle-shaped crystals contained calcium. The mechanism by which the psammoma bodies are formed has not been clearly explained. The above findings support the view that the psammoma bodies may occur initially in the cytoplasm of the tumor cells, followed by their release probably in a way of active excretion.