Hyperornithinemia due to ornithine aminotransferase (OAT) deficiency is associated with gyrate atrophy of the choroid and retina. Patients usually become virtually blind by age 55. Treatment includes a low arginine diet and pharmacological doses of pyridoxine (vitamin B₆). Long term compliance to an arginine-restricted diet, especially when started at a young age, can slow the progression of the chorioretinal lesions and loss of vision. Rarely, neonates can present with hyperammonemic encephalopathy, hypoargininemia, and hypoornithinemia and require arginine supplementation. In the hyperornithinemia, hyperammonemia, and homocitrullinuria (HHH) syndrome there is a wide spectrum of clinical manifestations, most of which are related to the toxicity of hyperammonemia. Progressive spastic paraparesis is often a late complication. Patients have a marked elevation of plasma ornithine associated with hyperammonemia and increased urinary excretion of homocitrulline. HHH results from a defect in the import of ornithine into the mitochondrion and consequent urea synthesis malfunction. Treatment is similar to that for patients with urea cycle disorders.