Purpose: To determine the cost effectiveness of intraperitoneal versus intravenous regimens for adjuvant treatment of optimally resected stage III ovarian cancer. Patients and Methods: A decision model was developed to compare the cost effectiveness at 7-, 11.5-, and 35-year horizons of intravenous carboplatin and paclitaxel (IV-CARBO/PAC), intravenous cisplatin and paclitaxel (IV-CIS/PAC), or intravenous paclitaxel followed by intraperitoneal cisplatin and paclitaxel (IP-CIS/PAC). Survival data were from women participating in representative Gynecologic Oncology Group (GOG) protocols. Medicare reimbursement rates and the Agency for Healthcare Research and Quality Database were used to estimate costs for treatment regimens and grade 3 to 4 adverse effects, respectively. Results: Median predicted survival was 66, 57, 51, and 48 months for IP-CIS/PAC, IV-CARBO/PAC, IV-CIS/PAC (GOG 172), or IV-CIS/PAC (GOG 158), respectively. Across a range of analyses, IV-CIS/PAC was more costly and had lower life expectancy than IV-CARBO/PAC. Compared with IV-CARBO/PAC, IP-CIS/PAC had an incremental cost-effectiveness ratio (ICER) of $180,022 per quality-adjusted life year (QALY) saved at a 7-year time horizon, $71,835/QALY at 11.5 years, and $32,053/QALY over a lifetime. Extending the survival advantage of IP-CIS/PAC over 11.5 years and a lifetime results in ICERs of $26,249 and $23,973, respectively. Assuming IP-CIS/PAC and IV-CIS/PAC were equally effective when administered on an outpatient basis, the ICER of IP-CIS/PAC compared with IV-CARBO/PAC was $26,311. Conclusion: Inpatient IP-CIS/PAC, while not cost effective compared with IV-CARBO/PAC at 7 years, becomes cost effective if a longer time horizon is modeled and/or a survival benefit can be assumed to persist longer than currently available data. Outpatient IP-CIS/PAC may also be cost effective compared with IV-CARBO/PAC if proven as effective as inpatient IP-CIS/PAC.