 The gastrointestinal string sign represents a severe narrowing of loop of bowel, in which a thin stripe of contrast within the lumen looks like a string.It may be seen in Crohn's disease, hypertrophic pyloric stenosis, carcinoid tumor and colon cancer.In people with Crohn's Disease, the string sign is caused by incomplete filling of the intestinal lumen, which results from irritability and spasm associated with severe ulceration.