Background: PD-1 activation by its ligands (PD-L1 and PD-L2) inhibits T-cell activation and plays a role in cancer progression. PD-L1 is widely expressed in many cell types in tumor microenvironment. In contrast, expression of PD-1 is restricted to a small subset of T-lymphocytes. Inhibition of PD-1/PD-L1 interaction showed no benefit in a small number of colorectal cancers (CRC) studied in clinical trials. We investigated tumor infiltrating PD-1+ lymphocytes and PD-L1 expressing cells in CRC to gain insight in their role as biomarkers. Methods: 77 CRC cases (53 sporadic and 24 hereditary, including 23 Lynch syndrome and 1 FAP) were profiled (Caris Life Sciences) for the presence of PD-1 and PD-L1 expressing cells, mismatch repair proteins, DNA microsatellite instability (MSI) and select cancer genes sequences (NGS). Only intraepithelial PD-1+ lymphocytes (IEL) and aberrantly expressed PD-L1 on carcinoma cells were considered specific. Results: PD-1+ IEL were detected in 47% of sporadic CRC. Microsatellite stable (MSS) cancers were frequently (61%) negative for PD-1. Microsatellite instability-high (MSI-H, both Lynch syndrome and sporadic) were significantly (p<0.03) more frequently infiltrated with PD-1+ IEL than MSS (72% MSI-H vs. 39% MSS). Similarly, PD-L1+ cancer cells were more common in MSI-H (56%) than in MSS (21%, p=0.007), but the expression was patchy in all cases. Concurrent PD1+ IEL and PD-L1 cancer cells were seen in 30% of MSI-H and 5% of MSS cancers (p=0.008). Conclusions: Consideration of immune checkpoint therapies for colorectal cancer needs to consider the presence of PD-1 lymphocytes and cancer cell specific PD-L1 expression. PD-1+ IEL and PD-L1+ cancer cells are more frequent in MSI-H than in MSS colorectal cancers, which are rare in general CRC population.