Diverticular colitis is the term used to describe a particular pattern of active chronic inflammation in the sigmoid colon affected by diverticular disease, namely the occurrence of luminal mucosal inflammation, whether or not there is evidence of inflammation within and/or around the diverticula themselves. The pathogenesis remains uncertain but is almost certainly multifactorial. In some cases mucosal prolapse, faecal stasis and relative mucosal ischaemia have been implicated as important pathogenetic factors, whilst other cases are clearly the result of a mass effect caused by subserosal peridiverticulitis and suppuration. Symptoms and endoscopic findings are diverse. Histologically, the disease may vary from modest inflammatory changes with vascular ectasia, through classical mucosal prolapse changes, to florid active chronic inflammation, closely mimicking chronic inflammatory bowel disease, especially ulcerative colitis. Thus, accurate clinical and endoscopic correlation is vital for the attainment of the correct diagnosis. Diverticular colitis may respond well to treatment similar to that used for chronic inflammatory bowel disease, adding to the similarities of this disease, notably localised to the sigmoid colon, and ulcerative colitis. Indeed, in a few cases described in the literature, diverticular colitis may 'progress' to otherwise classical ulcerative colitis, suggesting, in some cases at least, a similar pathogenesis.