Four cases out of 204 new cases of colorectal carcinoma seen in a 24 month period were proven to have concomitant proximal ischaemic colitis. Ischaemic colitis associated with obstructing carcinoma of the colorectum may present dramatically with gangrene or colonic perforation if the acute vascular insufficiency is severe. Less severe degrees of ischaemic insult must be recognised intra-operatively as the incorporation of ischaemic colon in a colonic anastomosis may result in an anastomotic leak. Two other patients with ischaemic colitis were wrongly diagnosed as colorectal carcinoma on colonoscopy. Endoscopic features of ischaemic colitis may mimic colonic carcinoma and endoscopic biopsy is essential where any doubt exist as to the possibility of ischaemic colitis so that unnecessary surgery may be avoided.