In a 25-year-old man with adult-onset type II citrullinemia, liver transplantation resulted in elimination of hyperammonemia and amino acid abnormalities associated with the disease. Postoperatively, a high intensity area in the right cingulate gyrus on a T2-weighted brain magnetic resonance imaging (MRI) also disappeared, suggesting that it reflected an early reversible lesion due to the hyperammonemia. Moreover, the serum level of pancreatic secretory trypsin inhibitor (PSTI), which had been elevated, was normalized. Since the levels of PSTI mRNA and PSTI have been reported to be increased in the livers of type II citrullinemia patients, measurement of serum PSTI levels could aid in the diagnosis of this disease.