Background: Chemoradiotherapy (CRT) remains the standard treatment choice for locally advanced rectal cancer (LARC). Neoadjuvant chemotherapy alone with doublet mFOLFOX6 (folinic acid, 5-fluorouracil, and oxaliplatin) seemed not to influence recurrence free survival with the advantage of less treatment-related complications. This phase II trial was designed to evaluate the efficacy and safety of neoadjuvant triplet chemotherapy with mFOLFOXIRI (folinic acid, 5-fluorouracil, oxaliplatin, and irinotecan) in patients with LARC (NCT03443661). Methods: Patients with LARC received up to 5 cycles of mFOLFOXIRI (irinotecan 150 mg/m², oxaliplatin 85 mg/m², leucovorin 200 mg/m² were administered on Day 1, fluorouracil 2400 mg/m² was administered as a continuous intravenous infusion for 48 hours on Day 1, and was repeated every 14 days). Magnetic resonance imaging (MRI) was performed to assess the baseline and post-chemotherapy TN stage. Radical resection was performed within 4–6 weeks of the last dose of chemotherapy if the tumor shrank or remained stable. Adjuvant chemotherapy with mFOLFOX6 or XELOX (oxaliplatin and capecitabine) was recommended. Postoperative radiation was planned for R1 resection, ypT4b, ypN2, and positive circumferential resection margin (CRM). The primary endpoint was the pathological complete response (pCR) rate. Results: Between December 2015 to March 2019, a total of 50 patients were enrolled. 48 (96%) of the patients were clinically node-positive, 28 (56.5%) were CRMnvolved, and 39 (78.4%) were extramural venous invasion (EMVI)-positive. The median cycle of neoadjuvant mFOLFOXIRI chemotherapy was 5 (range, 1–5). A total of 46/50 (92%) patients underwent total mesorectal excision (TME) surgery, all with R0 resection. The pCR rate was 4.3% (2/46). Twenty-three of 46 (50%) patients achieved pathological node-negative status. The proportion of pathologically positive CRM and EMVI were 2.2% and 34.7%, respectively (table). Adjuvant radiotherapy was given to 14/46 (30.4%) patients. The most common Grade 3 or more toxicities included neutrocytopenia (50%), leukopenia (14%), and diarrhea (12%) during neoadjuvant chemotherapy. Clinical meaningful surgical morbidities included pneumonia (n=1), pelvic infection (n=1), and anastomotic fistula (n=1). With a median follow-up time of 33 months (range, 14–73 months), local recurrences and distant metastases were confirmed in 3 (6.5%) and 8 (17.4%) cases, respectively. Conclusions: Neoadjuvant chemotherapy with mFOLFOXIRI yielded a significant down-staging effect with an ordinary pCR rate, and seemed effective in eliminating EMVI and transforming CRM-positive to CRM-negative status in patients with LARC. The preliminary survival results are promising. This regimen could serve as a potential alternative to CRT in selected patients with LARC. Clinical trial information: NCT03443661