Toxic megacolon is a well known complication in inflammatory bowel disease such as ulcerative colitis or Crohn's disease. The development of toxic megacolon as a complication of infectious colitis is rare. However it is recognised as a complication of enteric infections caused by Clostridium difficile, Campylobacter jejuni, Shigella, Salmonella species, Cytomegalovirus and amoebae. We describe a case of necrotising haemorrhagic ileo-colitis in a previously fit and healthy young adult female caused by Escherichia coli O157 where toxic megacolon developed as a complication along with hemolytic uremic syndrome (HUS).