Treatment of inflammatory bowel disease (IBD) generally relies on long-term use of anti-inflammatory and immunosuppressive agents. The adverse effects of those drugs make it important to prescribe the minimal regimen that is effective. An objective method for noninvasively quantifying severity of bowel inflammation would thus be valuable in guiding inflammatory bowel disease therapy. Using positron emission tomography (PET), we show that white blood cells (WBCs) labeled with 2-deoxy-2-[18F]fluoro-D-glucose (FDG) can serve as a quantitative marker for identifying the presence and severity of intestinal inflammation. In both murine and human subjects, PET images of FDG-labeled WBCs demonstrated little tracer uptake in healthy gastrointestinal and urinary tracts, where physiologic distribution of FDG images of glucose metabolism often compromises abdominopelvic PET imaging of intestinal pathology. Intestinal foci of FDG-labeled WBCs were confirmed to represent inflamed bowel through histopathologic or colonoscopic analysis, and intensity of foci measured in PET images correlated well with histopathologic measures of degree of inflammation. FDG-labeled WBC's, in conjunction with PET, can be used to provide quantitative assessment of bowel inflammation noninvasively, accurately, and rapidly.