Only mismatch repair (MMR)-deficient colorectal cancer (CRC) appears to respond well to PD-1 inhibition at the present time. Emerging evidence suggests a role for micro-environmental factors such as CD25+ve cells modulating response to PD-1 inhibition. In the ApcMin/+ model of familial adenomatous polyposis (MMR-proficient CRC), increased Cyclooxygenase-2 (Cox-2) expression by alternatively-activated mononuclear phagocytes promotes intestinal tumorigenesis by mechanisms which may include immune suppression. To gain insight into this, we compared regulatory T-cell (Treg) populations between ApcMin/+ and wild-type mice prior to and after the phase of increased intestinal Cox-2-dependent PGE2 production. There was no difference in systemic Treg function or numbers between ApcMin/+ and wild-type mice. However, increased numbers of small intestinal CD25+ve Tregs were observed with increased Cox-2 activity in the absence of any difference in the expression of Tgf- or Tslp between ApcMin/+ and wild-type mice. Cox-2 inhibitor therapy (Celecoxib) reversed the increase in ApcMin/ + intestinal CD25+ve Treg numbers, without decreasing numbers of CD25+ve systemic Tregs. Foxp3+ve and Cox-2+ve cells were co-localized to the interstitium of adenomas of ApcMin/+ mice. These results suggest selective dependence of an ‘activated Treg’ phenotype on paracrine Cox-2 activity in ApcMin/+ small intestine. For therapeutic potential, further studies are required to evaluate the relevance of these findings to human cancer as well as the functional significance of CD25+ve intestinal Tregs in cancer.