The statistical analysis documented in this article was performed to determine the accuracy of mammography in diagnosing a palpable cancer of the breast. All of the women studied underwent mammography after the discovery of the cancer, which was proven by biopsy; the cancer did not involve the skin. All mammograms that demonstrated any abnormality in the area of the cancer were deemed to be positive. This study demonstrates that 22% of the women who have a palpable cancer of the breast will have a false-negative mammogram; 44% will be women of 50 and younger, and 13% will be women more than 50 years of age. Mammography is detrimental if used to avoid biopsy when there is a persistent mass, especially in younger women. It is concluded that biopsy should be performed on every undiagnosed, persistent, dominant mass without consideration of the mammogram.