National guidelines encourage counseling high-risk women about pharmacologic breast cancer risk reduction. We evaluated the use of integrated health care data to identify and characterize breast cancer chemoprevention use. Chemoprevention included US Food and Drug Administration-approved use of tamoxifen and raloxifene and off-label use of aromatase inhibitors (AIs). Using electronic medical and pharmacy records (EMRs) in the Kaiser Permanente Northern California health care system, we sampled cancer-free women age 35 to 69 years who used tamoxifen, raloxifene, exemestane, anastrozole, or letrozole from 2005 to 2013. Risk-benefit profiles were calculated for tamoxifen and raloxifene using published indices. The proportion of days covered was calculated from pharmacy records to assess adherence. Among 90 chemoprevention users (confirmed with EMR review from a sample of 371 women), 74% used tamoxifen, 11% used raloxifene, and 13% used an AI. For tamoxifen and raloxifene users, the risk-benefit index indicated 23% of women had insufficient evidence that benefits would outweigh risks. For all agents, adherence decreased from an average proportion of days covered of 75% at 1 year to 67% at 5 years. Automated EMR searches identified breast cancer chemoprevention users with 60% positive predictive value overall and 75% for tamoxifen after post hoc modifications. Our study contributes to an emerging picture of breast cancer chemoprevention use in real-world settings, where evidence of net benefit is not uniform and nonadherence is common. Among breast cancer chemoprevention agents, our automated selection best performed for tamoxifen use. We also identified off-label use of AIs for chemoprevention, suggesting that expansion of risk-benefit indices to include AIs is warranted.