It has been suggested that smoking is associated with estrogen-negative breast cancer but not with estrogen-positive breast cancer. A population-based case-control study was conducted in Geneva, Switzerland, to determine the relation of passive and active smoking to breast cancer when the referent unexposed category consisted of women unexposed to active and passive smoke. The 242 patients with breast cancer (cases), in whom estrogen receptor (ER) status was determined on biopsy material, were compared with 1,059 women free of breast cancer (controls). Lifetime history of active and passive smoking was recorded year by year, between age 10 and the date of interview. Prevalence rates of ER+ tumors were 74.7% in pre-menopausal women and 74.2% in post-menopausal women. Post-menopausal active smokers had a lower prevalence of ER+ tumors (70.0%, p = ns). Among pre-menopausal women, the age-adjusted ORs of breast cancer with having smoked an average of > or = 20 cigarettes per day (cpd) during lifetime were 2.7 for ER- tumors and 2.6 for ER+ tumors. Among post-menopausal women, corresponding ORs were 5.7 for ER- tumors and 2.4 for ER+ tumors. Smoking was related to both ER- and ER+ breast cancer in pre- and post-menopausal women, but the strength of the association appeared to be greater for ER- tumors among post-menopausal women.