The principal goal of therapy when liver transplantation is used for the treatment of metabolic disease is to correct the metabolic error. By doing so, liver transplantation eliminates the hepatic and peripheral consequences of the error. Inborn errors involving the urea cycle appear on theoretical grounds to be amenable to treatment using liver transplantation and, indeed, published data demonstrate that this approach to therapy can be successful. The purpose of this study is to examine the outcome of liver transplantation done for the indication of urea cycle defects in a large group of patients. The first goal of the study is to determine with certainty that liver transplantation corrects hyperammonaemia and halts the progress of disease. A second goal is to determine the extent of neurological recovery in children previously injured by hyperammonaemia. The final goal is to understand whether the quality of life is improved and medical expense is reduced by transplantation. The study involved a survey of major transplantation centres. Four centres provided data about 16 patients, 14 of whom were alive 11 months to 6 years after transplantation. The results demonstrate that liver transplantation resulted in correction of hyperammonaemia in all patients. The neurological outcome after transplantation correlated closely with the condition prior to transplantation. This population of patients has had relatively few problems in the long term related to the liver transplant itself. The quality of life seems to be much improved, but further study will be needed to confirm this. Limited data involving two patients show a reduction in the cost of care. We conclude from our experience that liver transplantation can be an effective treatment for children with urea cycle defects.