Although several risk factors are common to
endometriosis and
breast cancer, the results of observational studies of an association have so far been inconsistent. We evaluated the relationship between
endometriosis and
breast cancer on the basis of data on selected
cancers and medical histories from the Danish nationwide
cancer and hospital registries used in a large case-cohort study. A total of 114,327 women were included in the study of whom 1,978 women had received a diagnosis of
endometriosis and 16,983 had had a diagnosis of
breast cancer between 1978 and 1998. Of the women with
endometriosis, 236 subsequently received a diagnosis of
breast cancer. The crude overall rate ratio for
breast cancer after
endometriosis was 1.00 and after adjustment for reproductive factors, calendar-period, bilateral
oophorectomy and benign
breast disease, the rate ratio was 0.97 (95% confidence interval, 0.85-1.11). The risk for
breast cancer increased with age at diagnosis of
endometriosis, so that women in whom
endometriosis was diagnosed at a young age (approximately <40 years) had a reduced risk for
breast cancer and women in whom
endometriosis was diagnosed at older ages (approximately > or =40 years) tended to have an increased risk for
breast cancer. The reduced risks observed among young women may reflect their exposure to drugs with antiestrogenic effects. The increased risk associated with
endometriosis among postmenopausal women may be due to common risk factors between postmenopausal
endometriosis and
breast cancer or an altered endogenous
estrogen.