SUMMARY We have investigated the correlation of 24 h and 48 h faecal Indium-lll excretion with each other and with several clinical activity indices for Crohn's disease (CD): Crohn's disease activity index (CDAI), activity index (AI), simple index (SI), Oxford score, and laboratory parameters, such as ESR, serum albumin, orosomucoid, C-reactive protein, alpha-l-antitrypsin (at,-AT) faecal concentration, and al-AT clearance in 58 CD patients (37 with small bowel and 21 with colonic disease). A significant correlation was found between 24 and 48 h faecal Indium-111 excretion for small bowel (r=0.708, p<00001) and colonic disease (r=0.994, p<00001). The median faecal Indium-111 excretion for colonic involvement (4%; 0-15-50% median and range) was significantly (p<00005) higher than that for small bowel disease (0.45%; 0.03-29%). No significant correlation was found between faecal Indium-111 excretion and any activity index in the patients with small bowel disease, while in the group of patients with colonic localisation only the AI showed a significant correlation (r=0-593, p<002). Faecal Indium-ill excretion was significantly correlated with al-AT clearance (r=0.712, p<00001) and faecal ctl-AT concentration (r=0'750, p<00001) in small bowel and in colonic localisation (r=0-530, p<0O02 and r=0.444, p<005). Serum albumin was significantly correlated only in the group of patients with colonic disease (r= -0.593, p<005). The present study shows poor agreement between activity indices, serum parameters of activity and faecal Indium-111 excretion. As a good correlation was found with the al-clearance, which reflects losses into the gut, these results may suggest that faecal Indium excretion does not only reflect activity of inflammation, but may relate to the extent of intestinal ulceration.Crohn's disease is a chronic relapsing disease in which the clinical presentation varies considerably. Difficulties arise in accurately assessing the disease activity which is important in the management of the patient and in analysing prospective trials. Laboratory variables such as erythrocyte sedimentation rate (ESR), serum albumin, C reactive protein and orosomucoid concentrations have been used but are not always reliable because they lack specificity for Crohn's disease.