A ten-year-old quarter horse mare presented to Cornell University’s Equine and Farm Animal Hospital in September of 2009. The mare had a one-day history of colic signs. On presentation, the mare demonstrated severe neurologic deficits including corticol blindess and ataxia. Blood was collected for a blood ammonia concentration, which demonstrated moderate hyperammonemia. Further tests established normal liver function. The horse was treated for presumptive idiopathic hyperammonemia. Idiopathic hyperammonemia is rarely diagnosed and is commonly associated with colic episodes preceded by neurologic signs. The exact etiology of idiopathic hyperammonemia is unclear but likely involves a disruption of the microflora of the gastrointestinal tract or compromise to the bowel wall. Prognosis is generally guarded. The mare in the current case survived with supportive care in the form of IV fluids, oral neomycin, antibiotics, and Vitamin B complex. This case report will explore the details of this case as well as discuss the pathogenesis, diagnosis and medical management of idiopathic hyperammonemia.