Microscopic colitis (MC) constitutes the main entities collagenous colitis (CC) and lymphocytic colitis (LC), diseases that are relatively recently described (in 1976 and 1989, respectively).

The aims of this thesis were to study the epidemiology of MC, to describe how these diseases affect patients in terms of symptom burden and health-related quality of life (HRQoL), to study potential risk factors such as familial factors, childhood circumstances, educational level, marital status, smoking and comorbidity, and to describe a cohort of patients with ulcerative colitis (UC) or Crohn’s disease (CD) and subsequent MC, and vice versa.

During 1999–2008 in Sweden, the mean annual incidence of MC was 10.2 per 105 inhabitants, compared with 5.2 per 105 inhabitants for CC, and 5.0 per 105 inhabitants for LC. The prevalence of MC on 31 December 2008 was 123 per 105 inhabitants. Women appeared to be especially affected – the female:male ratio was 3.6:1 in CC and 4.6:1 in LC.

Patients’ HRQoL is impaired both in active CC and in LC. Patients with CC in clinical remission have persisting symptoms: abdominal pain, fatigue, arthralgia and myalgia; LC patients in remission have persistent fatigue compared with controls. This illustrates that the longterm outcome is different in CC compared with LC.

Microscopic colitis is associated with a family history of MC, indicating that familial factors may play a role in the pathogenesis of this disease. We confirm earlier reports that smoking is a risk factor in MC.

In the present study population, CC was associated with rheumatic disease and previous appendicectomy. Moreover, CC and LC were associated with thyroid disease and coeliac disease and, interestingly, with a history of UC.

Most patients with UC or CD and subsequent MC, or vice versa, had UC or CD first and later developed MC. The majority had extensive UC and later onset of CC. Microscopic colitis should be considered in patients with UC or CD if there is onset of chronic watery diarrhoea without endoscopic relapse of mucosal inflammation.