Adult-onset type II citrullinemia is characterized by which deficiency of argininosuccinate synthetase (ASS) protein is found specifically in the liver of patients. Our recent study using differential messenger RNA (mRNA) display showed that the expression of human pancreatic secretory trypsin inhibitor (hPSTI) mRNA increases significantly in the liver of all type II patients tested. In the present work, we found that the concentration of hPSTI protein was higher in the liver of type II patients than controls. Because it is well known that PSTI is a secretory protein and because serum PSTI has been proposed as a marker of pancreatic and nonpancreatic diseases, we measured the hPSTI level in the blood of type II patients and compared it with other serum markers such as elastase 1, trypsin, phospholipase A2, alpha-fetoprotein, CA19-9, and C-reactive protein (CRP). We found a significant increase in serum hPSTI level with no change in the other serum markers. These results suggest that serum hPSTI is useful as a diagnostic marker for adult-onset type II citrullinemia.