Inborn error of metabolism of the urea cycle due to ornithine carbomoyltransferase (E.C. 2.1.3.3) deficiency associated with hyperammonemia and accumulation of ammonia in the brain and liver. In most cases the early symptoms appear within the first three days of life and include respiratory distress, feeding difficulty, hypotonia, lethargy, and death in untreated cases. Neonatal hyperammonemic coma lasting longer than 48 hours usually results in cortical atrophy and mental retardation. In late-onset OTC deficiency the symptoms appear from 2 months to 44 years with normal appearance at birth, followed by irritability, vomiting, lethargy, seizures, delayed development, ataxia, and seizures. Accumulation of ammonium in the brain and blood usually follows a protein load or intermittent infection. OTC deficient patients are particularly sensitive to toxic effects of valproate.