The clinical usefulness of serum pancreatic secretory trypsin inhibitor (PSTI) in pancreatic disease and gastric and colorectal cancer has been examined. The results showed that serum PSTI in acute pancreatitis was significantly higher than in normal subjects and it was also raised in acute exacerbations of chronic pancreatitis. Although the sensitivities of serum PSTI, amylase and elastase I were similar, serum PSTI in necrotizing hemorrhagic pancreatitis was 2.7 times higher than in mild acute pancreatitis. Only a few patients with chronic pancreatitis showed increased concentrations and the mean value was near normal. The mean PSTI in patients with pancreatic and colorectal cancer was higher than normal, although that of gastric cancer was within normal limits. The sensitivity of serum PSTI measurements in patients with these three malignant diseases was only about 30%. The results suggested that the measurement of serum PSTI could be useful in the diagnosis of acute pancreatitis, but of limited value in the diagnosis of other disease which we examined.