A new method of assessing disease activity in ulcerative colitis has been developed utilizing indium-111-labelled granulocytes. Faecal 111In excretion after intravenous administration of labelled granulocytes ranged from 1.1% to 45% of the injected dose in patients with ulcerative colitis, showing significant differences between mild, moderate, and severely active groups. There was a significant correlation between faecal 111In excretion and a clinical index based on the Crohn's disease activity index (r = 0.79, p less than 0.001) and the erythrocyte sedimentation rate (r = 0.73, p less than 0.001). Quantitative faecal 111In-labelled granulocyte excretion is an objective and specific method of assessing disease activity in ulcerative colitis.