Introduction: Colorectal cancer is one of the leading malignancies in Russia [1]. The standard approach for selected patients (pts) with locally advanced colon cancer is surgery with adjuvant chemotherapy. Several studies have shown that colorectal cancer (CRC) with presence of a disorder in the mismatch repair (dMMR) / microsatellite instability (MSI) is characterized with high sensitivity to the immune checkpoint inhibitors. Several studies have shown that MSI / dMMR CRC patients tend to be more responsive to immune checkpoint inhibitors such as pembrolizumab, nivolumab or ipilimumab. However, there was no information about the efficacy of prolgolimab, a PD-1 receptor blocking antibody. Prolgolimab was highly effective in melanoma treatment, while the toxicity was comparable to pembrolizumab and nivolumab. Methods: We initiated the phase II non-randomized open-label clinical trial. Inclusion criteria were: histologically verified, MSI / dMMR, clinical stage II–III CRC. According to study protocol, prolgolimab (1 mg / kg) is administered every two weeks, then surgery should be done after 6 months of immunotherapy (12 cycles). In case of surgical treatment refusal, the systemic treatment proceeds for 1 year. The co-primary endpoint was the complete response (pCR) rate. Secondary endpoints included tumor regression grade by Mandard (TRG), major pathologic response (MPR), overall response rate (ORR) disease free survival (DFS) and overall survival (OS). Here is a presentation of safety and pathologic response data — rates of pCR / MPR, objective response rate. Results: A total of 26 patients began treatment with prolgolimab from April, 2022 to February, 2024. Immune-related adverse effects of grade III–IV, were recorded in 1 (3,8 %) patient (autoimmune hepatitis grade IV); 4 (15,4 %) patients had adverse effects grade I–II: autoimmune thyroiditis, diarrhea, hypothyroidism. Two patients were refused to make a surgical treatment because of clinical CR and possible volume of surgery. Nine (34,6 %) patients underwent surgical treatment within 3 months after the immunotherapy completion: 7 patients had TRG 1 and pCR, 2 — TRG 2 and MPR after the treatment. ORR was 100 %, complete clinical response rate 40 %. The study is still ongoing, DFS and OS will be announced in further publications. Median follow-up time was 5 months. Conclusion: The first interim analysis data suggest a strong potential for neoadjuvant immunotherapy to become standard of care and allow further exploration of organ-sparing approaches in MMR / MSI CRC patients.