Background: Radiotherapy association with immunotherapy has a strong rationale. This study evaluates this combination before surgery in locally advanced rectal cancer (RC).Methods: R-IMMUNE (NCT03127007), a multicentric phase Ib/II prospective trial includes patients with stage II/III RC treated with a preoperative combination of radio-chemotherapy (45-50 Gy/25 fractions, 5FU 225 mg/m2/d, 5d/w from week 1-5) + atezolizumab 1200 mg/infusion (ATZ). The phase Ib had a 3+3 design with a safety period up to surgery and evaluated a single infusion of ATZ at week 3. The phase II, in progress, evaluates 4 infusions of ATZ at weeks 3, 6, 9 and 12. Surgery is planned at week 15. Primary objectives are safety and efficacy based on pathological complete response rate (pCR). Based on a 2-stage Simon design, 36 patients are needed in the phase II to detect a pCR rate increase from 15% to 35% (a ¼ 0.1 and b ¼ 0.1). At least 4 pCRs must be observed among 19 patients treated in the 1 st stage to move the 2 nd stage. abstracts Annals of OncologyVolume 32 -Issue S5 -2021 S537