The distribution of autologous leucocytes labelled with the gamma-ray emitting radioisotope indium-111 was studied in 25 patients with inflammatory bowel disease, 15 with Crohn's disease, and 10 with ulcerative colitis. In all instances the bowel lesion was identified. Radioactivity rapidly accumulated in the lesions over the first 2 h and then passed into the lumen of the bowel within the next 48 h, being redistributed from the liver and spleen. Normally, about 1% of the radioactivity passes into the stools in the 48 h after injection whereas approximately 20% of the administered radioactivity was found in the stools of the patients with inflammatory bowel disease. This noninvasive technique can show the extent of disease in patients too ill for conventional procedures. It can display abscesses and other lesions, and can be performed in outpatients.