A 1.5-mm gastrinoma of the duodenal wall was discovered during dissection of a duodenal ulcer in a patient in whom the Zollinger-Ellison (ZE) syndrome was later suspected due to gross autopsy findings. Multiple duodenal ulcers and gastric rugal hypertrophy were noted, and hyperplasia of the parietal cells associated with a duodenal gastrinoma was confirmed by immunohistochemical studies. In many cases of ZE syndrome, a primary neoplasm cannot be localized by angiography, computed tomography, ultrasound, or palpation at exploratory laparotomy. If a neoplasm cannot be identified, many cases will be attributed to islet cell hyperplasia, or antropyloric gastrin cell (G cell) hyperplasia. This case confirms that the primary neoplasm may be grossly undetectable and still produce the clinicomorphologic manifestations of this syndrome.