5028 Background: Endometrial cancer with adverse pathology has a significant risk of disease relapse if treated with surgery alone. We performed a prospective phase II trial testing the addition of concurrent chemotherapy during adjuvant radiation followed by adjuvant chemotherapy. Methods: Eligible patients had histologically confirmed endometrial cancer following a minimum of TAH-BSO, plus high-risk features of: node-positive disease, grade 3 FIGO 1988 stage Ic or IIb, or any stage III disease. Stage IV disease, serous or clear cell cancer was excluded. Treatment involved 54Gy of external beam radiotherapy (RT) to the pelvis ±extended field (EFRT) with carboplatin AUC 2 weekly × 6 during RT, followed by 4 cycles of carboplatin AUC 5-6 and paclitaxel 175 mg/m2 q3weeks. The primary endpoint was failure-free survival (FFS), with secondary endpoints of overall survival (OS) and toxicity. Results: 41 patients of median age 57 (range 31-79) were enrolled 06/04 to 08/08. 24 (59%) had stage IIIC disease with 11 (27%) having residual measurable disease on post-operative CT. All completed planned RT, including 11 (27%) treated with EFRT. 32 (78%) received all 6 doses of concurrent chemotherapy, 32 all 4 cycles of adjuvant chemotherapy and 28 (68%) received the planned 10 doses. After a median follow-up of 2.34 years, the estimated 3 year FFS rate was 73% (95%CI: 54-87%) and OS rate was 93% (95%CI: 76- 98%). By the study censor date, 9 patients (22%) had a recurrence with only 2 patients relapsing within the RT field. Treatment was well tolerated with no treatment-related deaths. Grade 3 or 4 nonhaematologic toxicity occurred in 14 with 12% grade 3 diarrhoea and 10% grade 3 arthralgia/myalgia. One pt respectively developed febrile neutropenia and grade 3 sensory neuropathy with 4 (10%) developing platinum-hypersensitivity. Conclusions: Adjuvant chemotherapy after chemoradiation for patients with high-risk endometrial cancer is a promising approach, though distant failure remains the dominant site of failure. This regimen was well tolerated but the high number of carboplatin doses resulted in a 10% hypersensitivity rate. Ongoing randomised trials should be supported to determine the optimum adjuvant therapy in endometrial cancer. Author Disclosure Employment or Leadership Position Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Bristol-Myers Squibb