The diagnosis of duodenal adenocarcinoma is still a complex gastrological and oncological problem. The described case illustrates the unusual character of the course, the reasonableness of the diagnostic procedures and the scope of the therapy undertaken. The causes of anemia, which was the first symptom of the disease, were not revealed in the tests carried out at this stage - gastroscopy, colonoscopy, with normal other basic laboratory tests. At the next episode, after 6 months from the first, the presence of a bleeding lesion within the duodenum was revealed. Further imaging and endoscopic examinations as well as the results of histopathological examinations from duodenum collected material were enabled the diagnosis of adenocarcinoma. In urgent mode, the operation was performed using the Whipplle method, removing the duodenum, the pyloric part of the stomach, the part of the pancreas head and the gallbladder. After 6 weeks, chemotherapy was applied in accordance with accepted standards, obtaining stabilization and gradual improvement of the general condition of the patient, which was confirmed in the performed tests.