Arginase deficiency is a very rare inborn error of the urea cycle. The first symptoms are often identified between 2 and 4 years and include clumsiness, spasticity and diminished growth. In untreated individuals symptoms are progressive resulting in loss of psychomotor function, spasticity, hyperactive deep-tendon reflexes, developmental delay, poor growth and seizures. Unlike the other defects of ureagenesis, patients with arginase deficiency rarely present in the neonatal period with acute episodes of hyperammonemia and, if they do, the episodes are generally less severe. A minority of patients have persistent or intermittent episodes of irritability, nausea, poor appetite and vomiting, which may progress to lethargy. Typically, hepatomegaly is present during acute episodes of hyperammonemia but is otherwise absent.