Background: Neoadjuvant CRT followed by surgery is the standard of care (SOC) for patients (pts) with stage II/III E/GEJ cancer. However, recurrence rates are high. Immunotherapy has demonstrated promising activity in advanced E/GEJ cancer. This trial evaluates safety and efficacy of perioperative A with CRT in resectable E/GEJ cancer. Methods: This is a 2-part phase I/II trial. Part 1 is a run-in phase with 6 pts for safety evaluation. Part 2 will enroll additional 18 pts in the expansion cohort. Pts with E/GEJ cancer of any histology receive CRT (41.4 Gy in 23 fractions) with carboplatin and paclitaxel as per SOC. Three doses of A (10 mg/kg, q14 days) are administered starting on day 29 of treatment, to coincide with the last chemotherapy dose. Surgery is performed ~8 weeks after CRT completion. Pts receive 6 doses of A after resection. Dose-limiting toxicity (DLT) evaluations are completed on the first post-operative clinic visit, 2-4 weeks post resection. Results: Between 6/2018 and 2/2019, 6 pts (all male, median age 62) enrolled in part 1: 6 adenocarcinoma (100%); 1 E, 3 Siewert 1, 2 Siewert 2; 1 cT2N0, 2 cT3N0, 3 cT3N1. All pts underwent successful resection with negative surgical margins. 1/6 pts had R1 resection due to tumor extension to inked adventitial surface without invasion of surrounding structures. There were no unexpected surgical complications. At resection, 2 pts had ypT0N0, 2 ypT1N0, 1 ypT2N0, and 1 ypT3N1 disease. Combination of CRT and A had an acceptable toxicity profile. No DLTs were seen in the first 5 pts, so expansion cohort is open to enrollment. No grade ≥3 immune-related AEs were observed. Immune-related hypothyroidism was seen in 1 patient (grade 2). 6/6 pts had reversible grade 3 or 4 lymphopenia; 1/6 had grade 3 neutropenia. Correlative studies are ongoing and will be presented at the meeting. Conclusions: Perioperative CRT with A is well tolerated with no unexpected toxicities. Additional safety and correlative data will be presented at the meeting. This study is actively enrolling pts to an expansion cohort at University of Wisconsin. Clinical trial information: NCT03490292