Some adult patients with microsatellite instability-high (MSI-H)/mismatch repair deficient (dMMR) small bowel adenocarcinoma might benefit from pembrolizumab in controlling the disease (i.e., some patients achieved a partial or complete response after treatment). These findings are based on 2 single-arm studies (i.e., no comparator) with fewer than 20 patients in each study, which limits the certainty of the findings. The longer-term benefit of pembrolizumab is unclear, as some outcomes (e.g., progression-free survival, overall survival) were not reached at the time of data analysis. The safety of pembrolizumab in patients with MSI-H/dMMR small bowel adenocarcinoma is unknown (no evidence was found for this population). No evidence was identified regarding the clinical effectiveness of pembrolizumab monotherapy for patients with MSI-H/dMMR appendiceal adenocarcinoma. No evidence was identified regarding the cost-effectiveness of pembrolizumab monotherapy for patients with MSI-H/dMMR small bowel adenocarcinoma or appendiceal adenocarcinoma. No evidence-based guidelines were identified regarding pembrolizumab monotherapy for patients with MSI-H/dMMR appendiceal adenocarcinoma. One guideline was identified that recommends pembrolizumab as an option for initial or subsequent therapy in patients with advanced or metastatic MSI-H/dMMR small bowel adenocarcinoma.