BACKGROUND: PD-1 blockade has been recommended as first-line therapy for nonresectable or metastatic mismatch repair-deficient/microsatellite instability-high (dMMR/MSI-H) colorectal cancer (CRC). However, the safety and efficacy of neoadjuvant PD-1 blockade immunotherapy for locally advanced dMMR/MSI-H CRC remain unclear. PATIENTS AND METHODS: From June 2020 to June 2022, 11 locally advanced dMMR/MSI-H CRC patients treated at the Sixth Affiliated Hospital of Sun Yat-sen University (Guangzhou, China) were enrolled. All patients received 6 sintilimab (Innovent, LTD) injections (200 mg/injection, every 3 weeks) before radical laparoscopic resection. The patient clinical and pathological data were analyzed retrospectively. RESULTS: dMMR was confirmed by immunohistochemistry for all patients. However, polymerase chain reaction (PCR) or next-generation sequencing confirmed MSI-H for only 90.9% (10/11) of the patients, while 1 patient had microsatellite stable (MSS) disease. After 6 injections of neoadjuvant anti-PD-1 therapy, 90.9% (10/11) of the patients (those confirmed to have dMMR and MSI-H disease) achieved pathological complete response (pCR). The other patient, who achieved major pathological response with residual tumor <1%, had dMMR but MSS disease. No grade 3 or above immunotherapy-related adverse events occurred [Common Terminology Criteria for Adverse Events ; version 5.0]. Overall, 72.7% (8/11) of the patients had grade 1-2 immunotherapy-related adverse events . No operational mortality or complications occurred within 30 days after surgery. CONCLUSION: Single-agent neoadjuvant PD-1 antibody immunotherapy was safe and effective in locally advanced dMMR/MSI-H CRC. Dual confirmation of MMR and MSI status by immunohistochemistry and next-generation sequencing or PCR is necessary for dMMR/MSI-H CRC patients before immunotherapy. The immunotherapy regimen used in this study deserves further validation in phase II and III clinical studies.