Serum pancreatic secretory trypsin inhibitor (PSTI) is considered to be one of the acute phase reactants which increase after tissue injuries like surgery or major trauma. 25 patients were studied to investigate the changes in serum PSTI during and after heart surgery. 18 patients underwent the operation with the aid of cardiopulmonary bypass (CPB)-CPB group, and the other 7 patients, all pediatric cases, were treated under simple deep hypothermia by surface cooling without CPB-simple hypothermia group. Serum amylase, serum C reactive protein (CRP), cardiac index and A-aDO2 after surgery were examined, and the duration of CPB or circulatory arrest were recorded. The correlation between serum level of PSTI after surgery and these parameters were also studied. During surgery, a transient elevation of serum PSTI was noticed in the period of CPB, but no significant change of serum PSTI was seen under simple deep hypothermia and circulatory arrest. On the first post operative day, a slight but significant increase of serum PSTI was seen in CPB group, on the other hand no significant increase was seen in simple hypothermia group. Postoperative increase of serum PSTI was correlated to circulatory arrest time in simple hypothermia group, and was inversely correlated to cardiac index determined immediately after the operation in CPB group. No correlation was seen between serum level of PSTI after surgery and serum amylase, CRP, A-aDO2 or duration of CPB. The magnitude of increase of serum PSTI after heart surgery was small, and did not seem to be well correlated to the extent of surgical insults.