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Background & Aims: Inflammatory bowel disease (IBD) refers to 2 chronic inflammatory diseases of the intestine, ie, ulcerative colitis and Crohn''s disease. IBD results from environmental factors (eg, bacterial antigens) triggering a dysregulated immune response in genetically predisposed hosts. Although the basis of IBD is incompletely understood, a number of recent studies have implicated defective innate immune responses in the pathogenesis of IBD. In this regard, there is much interest in therapies that activate innate immunity (eg, recombinant granulocyte-macrophage colony-stimulating factor). Methods: In this study, we screened expression and function of circulating leukocyte granulocyte-macrophage colony-stimulating factor receptor (CD116) messenger RNA and surface protein in 52 IBD patients and 52 healthy controls. Results: Our results show that both granulocyte and monocyte CD116 levels, but not CD114 or interleukin-3Rα, were significantly decreased in IBD compared to control (P < .001) and disease controls (irritable bowel syndrome; P < .001; rheumatoid arthritis; P < .025). IBD-associated CD116 repression was more prominent in patients with ulcerative colitis compared to Crohn''s disease (P < .05), was independent of disease activity (P > .05), and was not influenced by current medications (P > .05). Receiver operating characteristic curve analysis revealed that leukocyte CD116 expression is a sensitive (85%) and specific (92%) biomarker for IBD. Moreover, granulocyte CD116-mediated function (phosphorylation of signal transducers and activators of transcription 3) paralleled decreased expression of CD116 in IBD granulocytes compared to control (P < .001). Conclusions: These studies identify defective expression and function of CD116 as a distinguishing feature of IBD and implicate an associated defect in innate immune responses toward granulocyte-macrophage colony-stimulating factor. [Copyright &y& Elsevier]