Previous retrospective studies have suggested an association between consumption of non-steroidal anti-inflammatory drugs (NSAIDs) and the complications of diverticular disease. Ninety-two patients were entered into a prospective study of the complications of diverticular disease over a 3 year period; 31 were taking NSAIDs, compared with only four age- and sex-matched controls from a representative general practice (P less than 0.001). A second control group comprised 306 patients with cancer of the colon in whom NSAID consumption was again significantly lower than in patients with diverticular disease (22 of 306 versus 31 of 92, P less than 0.001). Of the 31 patients taking NSAIDs, 19 presented with a perforation or peritonitis. By contrast, only eight of the 61 patients not taking NSAIDs had such complications (P less than 0.001). Eleven patients presented with bleeding of whom five were taking NSAIDs and six were not. Patients admitted with complications of diverticular disease have a high incidence of NSAID intake, and it appears that NSAID consumption is associated with a more severe form of the disease.