Introduction: During the last decade, our institution saw a 260% increase in bilateral breast reconstruction cases, consistent with national trends. We reported a drop in average age of prophylactic mastectomy from 57 to 51 years. There is limited data on the likelihood of histological abnormalities in this population. This study measures the prevalence of occult histological findings in prophylactic mastectomy patients. Given the current healthcare reform climate, we estimate the lifetime cost implications of prophylactic mastectomy with immediate reconstruction vs. surveillance. Methods: A retrospective database of breast reconstructions at the Massachusetts General Hospital was searched from 2004 to 2011 for prophylactic mastectomy patients. Breasts with prior biopsy-proven LCIS, DCIS, or cancer were excluded. Patient demographics, risk factors, and pathology reports were collected. Lifetime treatment reimbursements were estimated with 2013 rates from the Center for Medicare and Medicaid Services using Medicare billing codes. Reimbursements were estimated for 45-year-old patients undergoing contralateral prophylactic mastectomy and 40-year-old patients undergoing bilateral prophylactic mastectomies, and then were compared to women opting for surveillance. Conversion rates to cancer in these patients were used to estimate the percentage patients in the surveillance groups that would need therapeutic mastectomy. Sensitivity analyses were done to test the robustness of the models. Results: 495 prophylactic mastectomy specimens were identified, of which 2.0% had invasive cancer, 4.4% had ductal carcinoma in situ (DCIS), and 10.9% had lobular carcinoma in situ (LCIS) as the highest-risk lesion. Only age group was predictive of finding DCIS or cancer (P=0.02). The likelihood of finding LCIS, DCIS, or cancer increased with age group (P<0.001) and decreased with prior bilateral salpingo-oophorectomy (BSO)(P=0.02). In almost all scenarios, lifetime reimbursements were lower for pursuing either contralateral or bilateral prophylactic mastectomy, with immediate single-stage implant, expander, or abdominal perforator free flap (DIEP) reconstruction, as compared to surveillance. Conclusions: Prophylactic mastectomy patients have a significant rate of occult histological findings, increasing with age group and decreasing with prior BSO. Lifetime cost estimates suggest a cost-saving role in bilateral and contralateral prophylactic mastectomies. Ultimately, such a critical decision needs to be made individually, but should not be hindered by cost concerns. This study addresses a gap in knowledge with broad interest, contributing evidence of oncologic risk and cost to help guide decision-making in prophylactic mastectomy.