Background: There are limited chemotherapeutic options for patients (pts) with advanced or recurrent endometrial cancer (EC). Reported objective response rates (ORR) for first-line doxorubicin/cisplatin/ paclitaxel combination therapy was 57%; with a median progression-free survival (PFS) of 8.3 months. The goal of this phase II study was to assess the efficacy and safety of pembrolizumab in combination with standard carboplatin/paclitaxel in pts with measurable advanced or recurrent EC. Methods: This was a single-arm, open-label, multi-center phase II study for pts with RECIST measurable advanced or recurrent EC coordinated by the Big Ten Cancer Research Consortium. Patients may have had received 1 prior platinum-based regimen, with a platinum free interval > 6 months, < one non-platinum chemotherapy, or prior hormonal therapy. Pts received carboplatin AUC 6, paclitaxel 175mg/m² (CT) and pembrolizumab 200mg IV every 3 weeks for up to 6 cycles; with dose reduced for prior radiation. The primary endpoint was ORR per immune-related RECIST. Planned sample size of 46 subjects provided 77% power to detect 15% ORR improvement compared to historical controls, with one-tailed test and 10% type I error rate. Results: 46 pts were enrolled. Median age was 67 (range: 43-86). 32 pts had recurrent and 14 had primary metastatic EC. Histological types were: 26 endometrioid, 11 serous, 3 clear cell, 6 other. 19 patients had received prior carboplatin/paclitaxel, 23 pelvic EBRT, 14 brachytherapy, 1 adriamycin and 1 hormonal therapy. Grade 3-4 adverse events (AEs) included: laboratory abnormalities (20), hematological (8), metabolism (6), nervous system (4), gastrointestinal (2), and others (6). There were 15 grade 3-4 SAEs occurring in 7 pts: vomiting (1), anaphylaxis (3), fever (2), dehydration (1), syncope (2), vascular (2), fatigue (1), neurological (2), thrombocytopenia (1), and no grade 5 SAEs. 36 patients were evaluable for response at the time of abstract submission. ORR was 77.8% (28/36) and median PFS was 10.55 months. Conclusions: The addition of pembrolizumab to standard of care CT chemotherapy for advanced or recurrent EC induced a clinically significant improvement in ORR compared to historical outcomes and toxicity did not exceed anticipated toxicity with standard treatment, supporting further testing in a phase III trial. Clinical trial information: NCT02549209