BACKGROUND: For average-risk women with unilateral breast cancer, contralateral prophylactic mastectomy (CPM) offers no survival benefit and contributes to increased costs and patient harm. Despite recommendations from professional societies against CPM, utilization of this service is increasing, partly due to patients' desire for breast symmetry when undergoing mastectomy. Most women with small tumors are candidates for breast-conserving surgery (BCS) and could avoid CPM. We describe CPM utilization in women with small, unilateral tumors, and identify determinants of possible overuse. METHODS: Using the National Cancer Database, we identified women with unilateral, T1 breast cancer. We evaluated utilization of BCS, unilateral mastectomy, and CPM and assessed patient, tumor, and facility factors associated with CPM. RESULTS: Of 765,487 women with small, unilateral breast cancer, 69% underwent BCS and 31% chose mastectomy. Of 176,673 women ≥70 y, 75% underwent BCS and 25% chose mastectomy. CPM rates in both cohorts have increased since 2006. Decreased adjuvant radiotherapy in older women was associated with increased BCS rates. Patient factors (younger age, white race, private insurance, and breast reconstruction), tumor factors (lobular histology, higher grade, and human epidermal growth factor receptor 2 positive/estrogen receptor negative status), and facility factors (type and geographic location) were associated with increased CPM rates compared with unilateral mastectomy in multivariable models. CONCLUSIONS: Most women with small unilateral breast cancer are candidates for BCS, yet one-third elects to undergo a mastectomy, of which a rising percentage opts for CPM. Tailoring deimplementation strategies to factors influencing treatment may help reduce CPM utilization and associated financial toxicity, pain, and disability.