The benign to malignant ratio (B:M ratio) among breast biopsies (number of benign breast lesions divided by number of breast cancers) is widely believed to be around 4:1 or 5:1. This belief appears to be based upon experience from the 1950's and 1960's. We investigated possible reasons for the current wide variability of the B:M ratio. A straight line relationship between the log B:M ratio and age exists in our data. The B:M ratio is sensitive to racial differences between patient groups but in only minimally affected by varied histologic inclusion criteria. The overall B:M ratio should not be used to counsel women about their breast cancer risk at the time of biopsy. Age-specific B:M ratios provide a more realistic risk assessment.