Bayes theory has been a basic method for making for making various decisions using many conditional and prior probablities. And it has been successfully used in medical fields such as EKG reading and determination of malignancy of a solitary pulmonary nodule. We try to modulate this theory in determination of malignancy of a palpable mass noted in mammography. The findings that have high malignant likelihood ratio are star-shaped margins, spiculated appearances, inhomogenous densities, malignant calcifications, skin changes, and retromammry space obliteration. And with these findings, the probabilities of malignancy are over 80% in prevalent age groups.