Chronic diarrhoea in adult horses can be very frustrating both for the veterinarian and the client. The horse is often bright with a good appetite which is quite different from the acute diarrhoea case where the horse is often dull, endotoxic and sick. A range of diagnostic tests can be performed and it is not uncommon that a diagnosis is still not reached. Findings are often non-specific and could be representative of many diseases. Symptomatic treatment can be used in cases of idiopathic diarrhoea to which horses often respond well. When inflammatory disease is the cause, the prognosis is often more guarded.