The assessment of disease activity in Crohn's disease involves determination of either clinical indexes (e.g., Crohn's disease activity index) or laboratory measurements (e.g., C-reactive protein and erythrocyte sedimentation rate). These have the disadvantage of being indirect and nonspecific correlates of gut inflammation. We have assessed disease activity in Crohn's disease by measurement of fecal leukocyte excretion after intravenous administration of either 111In-labeled mixed leukocyte or pure granulocyte preparations. With mixed leukocyte preparations, fecal excretion of radioactivity correlated with Crohn's disease activity index (r = 0.78, p less than 0.001) and C-reactive protein (r = 0.74, p less than 0.01). Using pure granulocytes, fecal 111In excretion (range 1.5%-52%) was much higher than with mixed leukocytes (range 0.1%-11.0%), showing significant correlations with Crohn's disease activity index (r = 0.731, p less than 0.001), C-reactive protein (r = 0.716, p less than 0.001), and erythrocyte sedimentation rate (r = 0.676, p less than 0.001). Quantitative fecal excretion of 111In-leukocytes is a new method of assessing disease activity in Crohn's disease, specific for bowel inflammation and suitable for objective assessment of disease activity in therapeutic trials.