Rationale: The usual metastases of breast cancer are mainly represented by the following organs: lymph nodes, bone, liver, lung, pleura and brain. While gastrointestinal involvement remains rare. Patient Concerns: A 67-year-old postmenopausal woman presented with abdominal pain and obstructive symptoms, she was diagnosed with duodenal metastasis of breast cancer with infiltration of the duodenal mucosa by lobular breast carcinoma. The patient had undergone the right breast mastectomy for infiltrating lobular carcinoma 6 years previously. Diagnosis: Metastatic infiltration of the duodenal mucosa by mammary lobular carcinoma was proven histologically and immunohistochemically. Lessons: The importance of thinking about metastases of the gastrointestinal tract in the presence of non-specific and vague symptoms and of clearly differentiating in this context these metastases from primary gastrointestinal tumors.