BACKGROUND: Breast cancer is the most frequent malignant tumour to metastasize into the gastrointestinal tract in female and is second only to malignant melanoma. Nevertheless gastrointestinal metastases arising from breast cancer are quite rare. The upper gastrointestinal tract is more frequently involved and lobular infiltrating carcinoma has a greater predilection compared to the ductal type. CASE PRESENTATION: The authors describe the case of a 70 years old woman with a preoperative diagnosis of gastric undifferentiated medullary--type carcinoma, which was the first manifestation of an occult breast carcinoma. The primary site of carcinoma was identified with the use of a panel of selected immunohistochemical markers. CONCLUSION: Our goal in this case report is to increase the awareness of surgeons and clinicians to rule out the possibility of mammary origin in circumstance of gastric cancer occurring in female, even in patients without a previous or concurrent history of breast carcinoma. Although not a particularly common event, it is, nevertheless, reported in the literature. The differentiation between primary gastric carcinoma and metastatic breast carcinoma is essential for planning the correct therapeutic approach, in order to avoid the patient unnecessary surgery.