The association between fibroadenoma and the use of exogenous hormones in women aged 18-74 years was examined in a case-control study conducted in Connecticut during 1979-1981. The study population included 251 women with biopsy-confirmed fibroadenoma and 1,081 control women drawn from inpatient and outpatient general surgical services of five Connecticut hospitals. Among women under age 45 years, oral contraceptive use was negatively associated with the occurrence of fibroadenoma (age-adjusted odds ratio (OR) = 0.57, 95% confidence interval (CI) = 0.42-0.79). The odds ratio for women over age 45 years who had ever used oral contraceptives was 1.65 (95% CI = 0.58-4.68). Women over age 45 years who had ever used replacement estrogens had an elevated odds ratio for fibroadenoma (OR = 2.83, 95% CI = 1.21-6.60). The data suggest either that the effects of exogenous hormones may differ by age, or that the changing composition of exogenous hormones has brought about different associations depending upon the cohort of women.