Ulcerative colitis (UC) involves inappropriate mucosal immune responses to intestinal microbiota. Gut dendritic cells (DC) are central immunoregulators of the response to commensal bacteria, and the subset of CD11c(+) cells within the human leucocyte antigen D-related (HLA-DR(+)) lineage (lin)(-/dim) population are activated in inflammatory bowel disease. We hypothesized that CD11c(-) cells within this population may also be involved in intestinal inflammation. HLA-DR(+) lin(-/dim) cells were identified in freshly isolated lamina propria mononuclear cells by multi-colour flow cytometry in 54 UC patients and 22 controls. Proportion and number of CD11c(+) and CD11c(-) cells, and surface expression of activation markers CD40, CD86, Toll-like receptor (TLR)-2, TLR-4, and CD56(+)[natural killer (NK) marker], were determined. Cytokine production was assessed by intracellular staining. Lamina propria colonic CD11c(-) HLA-DR(+) lin(-/dim) cells were increased significantly in inflamed and 'non-inflamed' UC tissue, compared with control tissue. CD11c(+) HLA-DR(+) lin(-/dim) cells were unchanged. Fewer CD11c(-) cells expressed activation markers and produced intracellular cytokines than their CD11c(+) counterparts, and they were weakly stimulatory in mixed leucocyte reactions. Few CD11c(-) cells expressed blood plasmacytoid DC markers, but a major subset expressed high levels of CD56. CD11c(-) cells decreased after inflammation resolved. Intestinal inflammation in UC is associated with the presence of cells that share phenotypic features of both DC and NK cells. This novel population of human colonic CD56(+) HLA-DR(+) cells may play a role in immune regulation or tissue repair. Their increase in quiescent UC may be a marker of subclinical inflammation.