Crohns disease causes pain in the abdomen and often diarrhoea and bleeding from the bowel. Crohns disease causes inflammation in the GI tract (anywhere from mouth to anus), which can be seen by passing a slim camera into the large bowel through the anus (colonoscopy) or through the mouth and down into the stomach (gastroscopy). Biopsies can be taken to confirm the diagnosis. The small intestine is less accessible but can be seen indirectly by barium meal Xray or by swallowing a wireless capsule that transmits pictures of the intestine to a data recorder worn on the waist. Blood tests showing inflammation (ESR and CRP) are usually raised when Crohns disease is active and deficiencies of vitamins, minerals and protein can occur and be detected by blood testing.

Patients with Crohns disease, particularly of the small intestine, may have fewer symptoms if they reduce the amount of high fibre foods they eat and also avoid nuts, sweetcorn, pith and pips. Maintaining a good intake of calories and protein is necessary since patients with Crohns disease have often lost a large amount of weight.