Ileostomy is a common component of surgical treatments for various gastrointestinal conditions. Loss of the fluid absorptive capacity of the colon results in increased fluid and electrolyte losses, which causes a state of relative fluid depletion. These losses can be offset in part by increased oral intake, but the remaining small intestine also compensates by increasing the efficiency of fluid and electrolyte absorption, a process termed , which occurs within weeks to months of ileostomy creation. Some patients fail to adapt adequately and have high ileostomy outputs from the time of surgery. Others with a previously well-adapted ileostomy may encounter periods of sustained high output when some additional process causes diarrhea. Many patients experience periods of high output after ileostomy creation and often require hospital readmission for this reason. Any patient with an ileostomy is at great risk of dehydration and electrolyte depletion should output rise dramatically. Prompt attention should be given to rehydration and identification of the underlying cause so that directed therapies may be implemented. This review discusses the alteration of normal intestinal fluid balance from colectomy with ileostomy, proposed mechanisms for adaptation, the differential diagnosis of ileostomy diarrhea, the evaluation of ileostomy diarrhea, and current treatment options.