A frequent problem with horses is respiratory noise during exercise somtimes associated with decreased performance. Among the possible causes are: dorsal displacement of the soft palate, collapse of the pharyngeal walls, and laryngeal hemiparesis. This article concerns itself with laryngeal hemiparesis. For more information on other causes of respiratory noise, their diagnosis, and treatment see the article Respiratory Noise.

The larynx is a cartilaginous box located at the back of the throat that leads to the trachea (windpipe). It is responsible for keeping food out of the trachea and vocalization. With laryngeal hemiparesis (LH) one of the two flaps responsible for closing the trachea when food is swallowed, becomes weak or even paralyzed and stuck in the closed position. At rest there is plenty of room for air to get through. As the horse's exercise level goes up he reaches a point where the flap impedes breathing. This is the cause of the noise and if severe, exercise intolerance. The degree of paralysis is variable and some horses have noise without exercise intolerance.

This article concerns itself with the cause, diagnosis, treatment options, and prognosis of this condition.

Noise during exercise is the most common sign and is called roaring. Exercise intolerance may be obvious in severe cases but in many cases the degree of exercise impairment is unclear.