Hyperargininemia is caused by deficiency of arginase 1, which catalyzes the hydrolysis of L-arginine to urea as the final enzyme in the urea cycle. In contrast to other urea cycle defects, arginase 1 deficiency usually does not cause catastrophic neonatal hyperammonemia but rather presents with progressive neurological symptoms including seizures and spastic paraplegia in the first years of life and hepatic pathology, such as neonatal cholestasis, acute liver failure, or liver fibrosis. Some patients have developed hepatocellular carcinoma. A usually mild or moderate hyperammonemia may occur at any age. The pathogenesis of arginase I deficiency is yet not fully understood. However, the accumulation of L-arginine and the resulting abnormalities in the metabolism of guanidine compounds and nitric oxide have been proposed to play a major pathophysiological role. This article provides an update on the first patients ever described, gives an overview of the distinct clinical characteristics, biochemical as well as genetical background and discusses treatment options.