We conducted a case-control study to search for any relationship between use of
oral contraceptives and development of
breast cancer or benign
breast disease. Women less than 50 years old with these diseases were matched with 2 controls by age, race, religion, and hospital. Home interviews elicited information on
oral contraceptive use and other host and environmental factors. The study population comprised 1,770 women, including 452 with
breast cancer and 446 with benign
breast disease. The relative risk of developing
cancer or benign disease was measured by matched set and summary chi-square analyses. Although the relative risk of developing
breast cancer among "ever-users" of
oral contraceptives was 1.1, the risk among women using
oral contraceptives for 2-4 years was 1.9 (significantly increased). This risk estimate reached 2.5 for the 2- to 4-year users if they were still taking
oral contraceptives when entered into study. Moreover, prior biopsy for benign
breast disease increased the
cancer risk among long-term users by as much as 11-fold. The relative risk of
breast cancer did not vary by age, interval since first use, earliest year of use, or interval since last use. These results could be interpreted to indicate that
oral contraceptives did not induce
breast cancer but may have accelerated the growth rate of preexisting
breast cancer. The relative risk of developing benign
breast disease among ever-users of
oral contraceptives was 0.8 (significantly reduced); it decreased with longer duration of use until it reached 0.2 for women who took these
hormones 8 years or more. The relative risk of benign breast was not affected by earliest year of use or interval since last use. We concluded that
oral contraceptives reduced the incidence of benign
breast disease, but that use of
steroid hormones is ill-advised for women with already established benign
breast disease.