Ulcerative colitis (UC) is a chronic inflammatory condition of unknown aetiology, characterized by a diffuse confluent mucosal inflammation of the colon starting from the rectum with a relapsing and remitting course. Conventional endoscopy was thought to be a reliable parameter of disease activity, but microscopic inflammation can persist despite normal mucosal findings. Histologically detectable inflammation is associated with a greater risk of subsequent relapse. A flare in UC activity is difficult to predict, but a simple, easily measured biological marker of relapse would be important in guiding the most appropriate therapy.