Duodenal carcinoma is a rare disease and is encountered in no more than 10% of cases of the Zollinger-Ellison syndrome. The article discusses a rare case in which an isolated duodenal gastrinoma was removed endoscopically with subsequent recovery from the Zollinger-Ellison syndrome. Study of the serum gastrin concentration with provocative tests allowed the Zollinger-Ellison syndrome to be diagnosed and the radicality of duodenal gastrinoma removal to be controlled. Retrograde duodenoscopy conducted after resection of the gastric stump by the Billroth II method localized the source of hypergastrinemia and made possible radical removal of the duodenal gastrinoma, which was confirmed by normalization of the blood serum gastrin concentration. The work includes a brief literature reference on the subject.