Metastatic carcinoid tumor of hindgut origin. A 50-year-old, asymptomatic woman with a retroperitoneal mass, found on routine physical examination, underwent surgery. Only formaldehyde-fixed material was available for ultrastructural studies. Despite poor preservation of subcellular details, tumor cells that were filled with round, electron-dense granules of the type found in hindgut carcinoid tumors were observed. Within 2 months of this surgery, a small primary rectal carcinoid tumor was discovered by endoscopy. Original magnification × 25,000. (Courtesy of Ann M. Dvorak, MD, Department of Pathology, Beth Israel Deaconess Medical Center, 330 Brookline Ave, Boston, MA 02215.)