A role for nitric oxide (NO) in ulcerative colitis has been suggested because mucosal NO synthase activity, measured by indirect assays, is increased in this disorder. We directly measured luminal NO in the colons of twelve controls and six patients with active ulcerative colitis. During colonoscopy, gas was aspirated from different parts of the colon and immediately analysed by a chemiluminescence technique. NO concentrations were more than 100 times higher in the patients than in the controls. Luminal NO is rapidly and easily measurable and may be used to monitor inflammatory bowel disease.