A 42-year-old man presented with a history of repeated episodes of consciousness disturbance for 5 years. The MRI showed abnormally high signal intensities on T2-weighted images at bilateral cingulate gyri, temporal lobes and insular regions, mimicking the finding of herpes simplex encephalitis. Hyperammonemia was disclosed. Serial work-up led to the diagnosis of adult-onset citrullinemia, deficiency of argininosuccinate synthetase. The clinical symptoms improved after diet control and medication. Follow-up MRI showed resolution of the abnormal signal intensities. The MRI findings of citrullinemia and other urea-cycle defects might be attributed to hyperammonemic encephalopathy, but the manifestations were varied. Similar distribution of the abnormalities in the MRI could be found in some reported cases and indicates probably vulnerable sites of hyperammonemic brain injury.