The value of the Crohn's disease activity index (CDAI) in defining clinical remission in Crohn's disease has been assessed in 71 studies using a new method to quantitate gut inflammatory activity: faecal 111In-labelled granulocyte excretion. The range of faecal 111In granulocyte excretion in the irritable bowel syndrome was found to be 0.2-1.9% (mean +/- SD 0.98 +/- 0.55%) of injected dose. 63 (89%) of studies with a CDAI less than 150 and 88% of studies with a serum albumin greater than 35 g/l had faecal 111In granulocyte excretion above the upper limit found in the irritable bowel syndrome ranging from 2.4% to 40%. This study shows that the majority of patients with Crohn's disease in clinical remission have significant gut inflammatory activity. Whether treatment of this activity will alter the natural history of the disease needs prospective evaluation.