Diverticular disease of the sigmoid colon is one of the most common diseases at sigmoidoscopy/colonoscopy and yet biopsies are not often taken from the colon afflicted by this disease. Indeed, biopsies are often contraindicated in acute diverticulitis because of the risks of perforation. Nevertheless, there is increasing understanding that the disease is associated with luminal mucosal pathology, namely diverticular colitis, which is a close histological mimic of chronic inflammatory bowel disease (IBD). Indeed, there is a close relationship between the diseases and there are occasional cases where diverticular colitis is followed, in due course, by the onset of true chronic IBD, specifically ulcerative colitis. The disease is also associated with characteristic mucosal polypoid change. Often mucosal biopsies show similar inflammatory changes to those seen in classical diverticular colitis but the pathology of mucosal prolapse may also be seen. The latter features may occur in many different situations, although all show the same characteristic morphological appearances. Such situations include stomas, both ileal and colonic, in association with ‘cap polyposis’, in solitary ulcer (mucosal prolapse) syndrome in the rectum, and at the anorectal junction in so-called inflammatory cloacogenic polyp. Several other conditions can produce mimicry of chronic IBD in mucosal biopsies, having in common the ability to produce underlying mass lesions that result in ‘secondary colitis’ in the mucosa. These conditions include endometriosis, pneumatosis coli, primary and secondary tumours, and inevitably, intramural and extramural suppuration associated with diverticular disease itself.