An investigation of the relationship between breast parenchymal patterns and breast cancer prevalence in a large referral population is presented. Mammograms were assigned to one of four categories according to our interpretation of Wolfe's classification. Cancer prevalence for the four patterns was similar when uncorrected for age,, and was very high compared to that in the general population. Under age 50, the prominent duct pattern (P2) was associated with a very high relative cancer risk and DY carried a smaller increased risk. After age 50, prevalences for the patterns were nearly equal. Relationship between these findings and epidemiology of breast cancer are discussed and suggestions made for utilizing parenchymal patterns to guide examination frequency.