Ornithine Transcarbamylase (OTC) deficiency is the most common disorder of the urea cycle. Males usually present in the neonatal period with massively elevated ammonia levels and hyperammonemic coma, encephalopathy and respiratory alkalosis. However, the clinical features of OTC deficiency are highly variable ranging from an acute neonatal presentation with a high mortality rate to asymptomatic adult males. OTC is an Xlinked trait. Approximately 20% of females who are heterozygous for mutations in the OTC gene are clinically symptomatic with disease severity similar to males with partial deficiency. Both heterozygous females and partially deficient males may present with late onset of symptoms, even into adulthood, with recurrent vomiting, history of protein avoidance, Reye-like syndrome, or with major neurologic impairment, neurobehavioral changes or seizures associated with hyperammonemia.