CA15-3 is a tumor marker associated with mammary tumors. Increased levels have been observed in patients with breast cancer, while normal low levels are usually found in women with no evidence of disease. The potential clinical uses of this marker include monitoring of patients with breast cancer, prognosis, and early detection of recurrence. Tumor markers are not usually used for diagnosis, as only in a few patients are elevated levels found. But when high levels of tumor markers are detected at an early stage, it may be of diagnostic value. In this study we describe a patient we believe to be the first in whom the CA15-3 tumor marker indicated breast cancer, while physical examination and the initial mammography were without suspicious findings. We also show sections of formalin-fixed, paraffin-embedded tissue stained by the CA15-3 indirect immunoperoxidase method.