Background: PD-1/PD-L1 blockade showed therapeutic efficacy in only microsatellite (MSI) colorectal carcinomas (CRC), however, the profile of PD-L1 and PD-1 expression in CRC is only partially described. Methods: We thus analyzed on FFPE whole-tissue sections of 80 CRC, the expression profile of PD-L1 by tumor and/or immune cells by immunohistochemistry (clone E1L3N) depending on the MSI status and the histological subtype, and correlated to the density of PD-1+ and Tbet+ (able to secrete IFNg known to induce PD-L1) tumor-infiltrating lymphocytes (TIL). Results: 78% of MSI CRC (32/41) overexpressed PD-L1 either by tumor or immune cells versus 46% of MSS CRC (18/39) (p 0.005). This overexpression was heterogeneous within the same tumor in most of cases. Among MSI CRC, PD-L1 was preferentially overexpressed in medullary carcinomas (MC, 19/21, 90%) compared with 65% (13/20) in non-medullary adenocarcinomas (p 0.06). PD-L1 expression by tumor cells was only observed in MSI CRC (19/41, 46% with PD-L1 expression in more than 5% of tumor cells – score 1), and preferentially in MC (57% vs 5% in no medullary adenocarcinomas, with PD-L1 expression in more than 50% of tumor cells – score 3, p 0.0005). Conversely, PD-L1 expression by immune cells was observed in MSI CRC (23/41, 56% with PD-L1 expression by more than 5 sheets of 50 positive cells) but also in MSS CRC (18/39, 43%) (p 0.5). The density of PD-1+ cells was significantly correlated to the PD-L1 expression, as well as the density of Tbet+ TIL. Conclusions: PD-L1 expression is 1) heterogeneous in CRC, among CRC but also within the same tumor, 2) preferentially observed in MSI CRC (78%), especially in MC (90%), where PD-L1 is expressed by tumor cells, 3) correlated with the density of PD-1+ or T-bet+ TIL, and 4) observed in a significant proportion of MSS CRC (46%) by immune cells only. From a clinical point of view, PD-L1 expression has to be determined at best in full tissue section and besides its preferential expression in MSI CRC, its significant frequency and expression profile (only by immune cells) in MSS CRC should be taken into account in the future clinical trials testing the efficacy of anti-PD-1/PD-L1 antibodies.