Ileostomy is a stoma (surgical opening) constructed by bringing the end or loop of small intestine (the ileum) out onto the surface of the skin, or the surgical procedure which creates this opening. Intestinal waste passes out of the ileostomy and is collected in an external ostomy system which is placed next to the opening. Ileostomies are usually sited above the groin on the right hand side of the abdomen.

Ileostomies are necessary where injury or a surgical response to disease has meant the large intestine cannot safely process waste, typically because the colon and rectum have been partially or wholly removed.

Diseases of the large intestine which may require surgical removal include Crohn's disease, ulcerative colitis, familial adenomatous polyposis, and total colonic Hirschsprung's disease. An ileostomy may also be necessary in the treatment of colorectal cancer or ovarian cancer. One example is a situation where the cancer tumor is causing a blockage (obstruction). In such a case, the ileostomy may be temporary, as the common surgical procedure for colorectal cancer is to reconnect the remaining sections of colon or rectum following removal of the tumor provided that enough of the rectum remains intact to preserve internal/external anal sphincter function.

In an end ileostomy, the end of the ileum is everted (turned inside out) to create a spout and the edges are sutured under the skin to anchor the ileum in place. Permanent ileostomies are usually done this way. An end ileostomy may be temporary, notably if some of the large intestine was removed and the bowel or overall health is not considered amenable to tolerating further surgery, such as an anastomosis to rejoin the small and large intestines.

In a temporary or loop ileostomy, a loop of the ileum is surgically brought through the skin creating a stoma, but keeping the lower portion of the ileum for future reattachment in cases where the entire colon and rectum are not removed but need time to heal. Temporary ileostomies are also often made as the first stage in surgical construction of an ileo-anal pouch, so fecal material does not enter the newly made pouch until it heals and has been tested for leaks—usually requiring a period of eight to ten weeks. When healing is complete the temporary ileostomy is then "taken down" (or reversed) by surgically repairing the loop of intestine which made the temporary stoma and closing the skin incision.

People with ileostomies must use an ostomy pouch to collect intestinal waste. People with ileostomies typically use an open-ended (referred to as a "drainable") one- or two-piece pouch that is secured at the lower end with a leakproof clip, or velcro fastener. The alternative is the closed-end pouch that must be thrown away when full. Ordinarily, the pouch must be emptied five to eight times a day. If the bag stays empty for more than four to six hours, individuals should contact their healthcare provider, as this may indicate intestinal blockage. The pouch and flange (both one and two piece pouches) are usually changed every 2–5 days.

Ostomy pouches fit close to the body and are usually not visible under regular clothing unless the pouch becomes too full. It is necessary to measure the stoma regularly as it changes shape after the initial surgery. The stomal- or colorectal-nurse does this initially for a patient and advises them on the exact size required for the pouch's opening. Changes in size and shape can indicate a problem and may signal a need to call a healthcare provider.

Some people find they must make adjustments to their diet after having an ileostomy. It is important for individuals to consult with their healthcare providers. Tough or high-fiber foods (for example: potato skins, tomato skins, and raw vegetables) are hard to digest in the small intestine and may cause blockages or discomfort when passing through the stoma. Chewing food thoroughly can reduce such problems. Some people find that certain foods cause annoying gas or diarrhea. Many foods can change the color of the intestinal output, causing alarm; beetroot, for instance, produces a red output that may appear to be blood. Nevertheless, people who have an ileostomy as treatment for inflammatory bowel disease typically find they can enjoy a more "normal" diet than they could before surgery. Correct dietary advice is essential in combination with the patient's gastroenterologist and hospital-approved dietician. Supplementary foods may be prescribed and liquid intake and output monitored to correct and control output. If the output contains blood, an ileostomate (patient) is advised to visit an emergency department.