Infiltrating monocytes and macrophages contribute to the initiation and perpetuation of mucosal inflammation characteristic for human inflammatory bowel disease (IBD). Peripheral blood monocytes expressing the low-affinity Fcgamma receptor CD16 have been identified previously as a major proinflammatory cell population, based on their unique cytokine secretion profile. However, the contribution of these cells to the pathogenesis of inflammatory bowel disease remains to be elucidated. Thus, in this study we investigated whether the peripheral CD16(+) monocyte count correlates with common IBD disease parameters, and whether these cells infiltrate the intestinal mucosa under inflammatory conditions. We observed that CD16(+) peripheral blood monocytes are increased significantly in active Crohn's disease, particularly in patients with high Crohn's disease activity index and colonic involvement. Furthermore, we found that CD16(+) cells are a major contributor to the inflammatory infiltrate in Crohn's disease mucosa, although their spontaneous migration through primary human intestinal endothelial cells is limited. Our data suggest that lamina propria, but not peripheral blood, CD16(+) monocytes are a crucial proinflammatory cell population in IBD, and a potential target for anti-inflammatory therapy.