Crohn's disease may present insidiously, especially in childhood, and diagnosis may be delayed. In addition, the clinical assessment of the extent of disease activity may be inaccurate. Using mannitol and lactulose as probe molecules we have carried out a cross sectional study of intestinal permeability in patients with active Crohn's disease (n = 17) and control subjects (n = 31). Activity was assessed by an activity index score. The lactulose:mannitol urinary excretion ratio was significantly increased in Crohn's disease. Overall sensitivity was 82%, and 92% when activity was moderate or severe. When permeability was compared with the activity index there was a significant correlation among patients. In five patients studied longitudinally a significant correlation was also present. Measurement of intestinal permeability is non-invasive, and may be useful both as a screening test in patients with non-specific symptoms, and for the assessment of the extent of disease activity in patients with Crohn's disease.