Two cases of occult breast cancer that we encountered are reported inaddition to the discussion on the literature. Case 1 was a 57-year-old woman who visited our department because she moted masses in the left axillary and supraclavicular fossae. When the mass in the left axillary fossa was examined via biopsy, it was diagnosed as metastatic adenocarcinoma. Because a mammogram showed very small calcification, but a possible malignant tumor in other organs was clinically denied, she was diagnosed as breast occult cancer. This patient underwent extended radical mastectomy of the left breast; the tumor was defined as T0, N3, M0 and stage IIIb. Case 2 was a 64-year-old woman who visited our department with the main complaint of an egg-sized painless mass in the left axillary fossa. Because the mass was diagnosed as metastatic adenocarcinoma via biopsy, her whole body was examined, a nd a mass, measuring about 2cm in diameter, was detected in the left thyroid lobe. While we were thinking of a possibility of breast occult cancer, we undetook operation based on suspected thyroid adenocarcinoma. Because that tumor was found to be benign goiter during operation, total mastectomy was performed at the same time. Her breast tumor was histopathologically defined as medullary tubular carcinoma with infiltration to a very limited extent.