The significance of inflammation of the mucosa of the ileal reservoir after restorative proctocolectomy is not known although in some cases it appears to be associated with symptoms when the condition has been referred to as pouchitis. This investigation has aimed to determined the prevalence of inflammation, to define pouchitis and to examine some factors which might be related to inflammation. Mucosal biopsies from the ileal reservoir were studied in 90 patients at up to 62 months after closure of the ileostomy. A histological grading system (0-6) was used to assess the severity of inflammation. Some degree of chronic and acute inflammation was found in 87% and 30% of cases respectively. The prevalence of a grade of 4 or more was 23% and 3.5%. There was a correlation between severity of chronic and acute inflammation. Severe histological acute inflammation (grade 4-6) was associated with sigmoidoscopic features of inflammation and with increased frequency of defaecation. Of 55 patients sigmoidoscoped by one clinician, 6 (11%) had pouchitis which was characterised by macroscopic inflammation of the reservoir, diarrhoea and a histological grade of 4 or more. The severity of chronic inflammation was not related to frequency of defaecation. Histological inflammation could not be correlated with the type of reservoir, residual volume after evacuation of a known volume of stool substitute introduced per anum into the reservoir or compliance of the reservoir. Acute inflammation was significantly more severe in patients with ulcerative colitis than in those with familial adenomatous polyposis.