Context Collagenous colitis and lymphocytic colitis, collectively designated microscopic colitis, have until recently been considered as rare gastrointestinal disorders. New data suggest, however, that these disorders are relatively common, and to reach the correct diagnosis both the gastroenterologist and the pathologist must be aware of these diagnoses when evaluating patients with persistent watery non-bloody diarrhoea. Starting point In an epidemiological study of a well-defined Swedish population, Martin Olesen and colleagues show that microscopic colitis needs to be considered as a common gastrointestinal disorder (Gut 2004; 53: 346-50). In colonic biopsy specimens from 1018 patients who had a colonoscopy because of non-bloody diarrhoea in 1993-98, 97 patients (9·5%) were found to have microscopic colitis. In about a third of these cases, the diagnosis was missed in the primary histological evaluation. Median age at diagnosis was 64 years for collagenous colitis and 59 years for lymphocytic colitis. The annual incidence of the diseases was higher than previously considered and matched the incidence of Crohn's disease, and in combination they approached the incidence of ulcerative colitis. Where next The high regional incidence of microscopic colitis means that both clinicians and pathologists need to be more aware of the diagnosis, especially in the elderly female population with a clinical picture of watery non-bloody diarrhoea. Because of the potentially disabling symptoms, clinicians need to develop and evaluate new therapies.