Antibiotic use has been associated with risk of breast cancer in previous reports. Using Cox proportional hazards analysis, we evaluated this association in 2,130,829 adult female subscribers of a health care program according to their receipt of prescriptions of antibiotics from outpatient pharmacies. Hormone use was taken into account. Altogether, 18,521 women developed breast cancer in up to 9.4 years of follow-up. Use of any antibiotic was associated with slightly increased risk [hazard ratio (HR), 1.14; 95% confidence interval (95% CI), 1.10-1.18] but there was little, if any, evidence of dose response, with HR of 1.17 (95% CI, 0.97-1.42) for >1,000 days of use compared with no use. The only two weakly associated antibiotic groups (HR >1.10 for >100 days of use) were tetracyclines and macrolides with HRs (95% CI) of 1.23 (1.11-1.36) and 1.16 (0.98-1.36), respectively. An association of lincosamides with breast cancer in an earlier, smaller database was not confirmed, but follow-up was too short in the present data for adequate evaluation. Medical record review suggested that acne and/or rosacea could be the underlying factor, associated with long-term antibiotic therapy and found by others to be associated with risk of breast cancer. Although causality cannot be ruled out, the observed associations of antibiotics overall, tetracyclines, and macrolides with breast cancer were weak and could be explained by uncontrolled confounding by the diseases being treated or by other factors.