Background: Perioperative chemotherapy improves cure rate in locally advanced gastroesophageal adenocarcinoma (GEA), and immune checkpoint inhibitors are active at the metastatic stage. This trial tests the hypothesis that the addition of avelumab to perioperative chemotherapy will increase the major pathologic response (MPR) rate in comparison with historical controls. Methods: Phase II study of avelumab + chemotherapy (docetaxel, cisplatin and 5-FU or mDCF) given every 2 weeks for 4 cycles before and after surgery. Main inclusion criteria: GEA, cT3 and/or cN+, M0, WHO PS 0-1. Main exclusion criteria: use of immunosuppressants, serious autoimmune disease, daily intake >10 mg prednisone. Staging studies: CT, PET-CT, endoscopic ultrasound, diagnostic laparoscopy. Surgical resection: D2 lymphadenectomy, en-bloc esophagectomy for type I/II gastroesophageal junction (GEJ) tumors. Aim of the study: MPR as defined as tumor regression grades 0-1 (modified Ryan scheme); as per hypothesis, this experimental regimen will result in a 20% rate of MPR, compared with 7% with chemotherapy alone. Simon 2-stage design: if less than 2 MPR are seen in the first 16 patients, the study will be closed. The study hypothesis cannot be rejected if at least 6 MPR are seen in the first 50 patients. All adverse effects are prospectively recorded per CTCAE guidelines in patients who have received at least one treatment cycle. Survival rates are calculated with Kaplan-Meier method. Preliminary results are presented since the study has met its primary endpoint. Results: Feb 2018-Feb 2020: 28 patients enrolled (25 M/3 F, age 45-78). Location: GEJ (23), stomach (5). Staging: cT3 (25), cT4 (1), cN+ (20). Biomarkers expression: mismatch repair (MMR) protein loss (3/28); PD-L1(clone 73-10) expression in 1% (TPS) or more of tumor cells seen in 12/28 samples, and >10% in 6 patients. Grade 3 toxicity: stomatitis (2/28); nausea (2/28); vomiting (1/28); diarrhea (1/28); hypothyroidism (1/28); arthralgia (3/28); neutropenia (1/28). Grade 4 toxicity: pneumonia (1/28); neutropenia (2/28). Postoperative 30-day mortality: 0%. One patient was excluded from efficacy analyses for M1 staging; 27 patients underwent surgery, 26 with R0 (96%). Six cases (22%) show MPR: 3 grade 0 (11%) and 3 grade 1 (11%) tumor regressions. No correlation was seen between MMR proteins or PD-L1 expression and tumor regression. With a median follow-up of 1.5 years (range 0.4-2.5), the disease-free survival rate is projected to be 0.92 (95% CI 0.83-1.00) at 12 months and 0.77 (95% CI 0.58-1.00) at 24 months. Conclusions: The combination of mDCF chemotherapy with Avelumab demonstrates a promising safety and activity profile. Ongoing laboratory investigations are underway to correlate our findings with tumor molecular features before exposure to treatment. Clinical trial information: NCT03288350