An extremely rare case of gastric cancer metastasized to the bilateral breasts is reported. A 46-year-old woman was admitted to the hospital because of a left breast tumor and back pain. There was a previous history of undergoing a gastrectomy for gastric cancer [ow (-), aw (+), P0, H0, n2, t2. mp, infγ, ly2, v0, stage IIIa, cure. B] on October 29, 1993, when the histopathologic type was poorly differentiated adenocarcinoma. The patient became having the back pain and left breast tumor since around May, 1996 and a bone scintigraphy revealed remarkable bone metastasis. In June, an 8.6×7.6cm elastic hard mass infiltrating into the skin and nipple in the entire left breast and left axillary lymph node metastases were palpated, when no tumor was present in the right breast. In October a 5.8×5.6cm tumor was also palpated in the right breast. Neither needle aspiration biopsy cytology nor incisional biopsy of the left breast could differentiate between signet ring cell carcinoma arisen from the breast and metastasis from the gastric cancer. So a left modi-fied radical mastectomy was performed. Histopathologic exploration of the resected material offered a suspicion of breast metastasis of the gastric cancer. In making the definite diagnosis, paradoxical concanavalin A staining, HIK 1083 staining and galactose-oxydase-Shiff reaction which revealed the same positive reactions as the gastric cancer tissue showed were useful. She died of lung metastasis and carcinomatous pleuritis 5 months after the mastectomy.