Mastocytic enterocolitis is a term describing the condition of chronic, intractable diarrhea in people with normal colon or duodenum biopsy results, but with an increased number of mast cells in the mucosa (the innermost layer of the colon). The increase in mast cells is not associated with systemic mastocytosis or cutaneous mastocytosis. The infiltration of mast cells is thought to be a reactive, gut-specific phenomenon from unknown stimuli. It is unclear whether the accumulation of mast cells causes, or is a response to, the inflammation that causes the symptoms. Most people with this condition respond well to treatment, which may involve medications such as cetirizine, ranitidine, cromolyn sodium, or corticosteroids.

Signs and symptoms of mastocytic enterocolitis primarily include chronic, intractable diarrhea and abdominal pain. Other symptoms that have occasionally been reported include constipation, nausea, and/or vomiting.

Mastocytic enterocolitis may be considered when a person has intractable diarrhea, and other causes have been ruled out, including systemic mastocytosis. It is diagnosed after an endoscopic procedure in which the doctor takes samples of tissues (biopsies) from the lining of the intestines. The tissue is then sent to a pathologist who looks at it under the microscope. Mastocytic enterocolitis is diagnosed when excess mast cells are present in the small bowel or the colon.

There is limited information in the medical literature about the treatment of mastocytic enterocolitis. Most people with this condition respond well when treated with medications such as cetirizine, ranitidine, and/or cromolyn sodium. Glucocorticoids also decrease mast cell concentration in affected tissues.