We measured serum immunoreactive interleukin 6 (IL-6) levels in patients after major thoraco-abdominal surgery and compared them with changes in serum C-reactive protein (CRP) and pancreatic secretory trypsin inhibitor (PSTI) levels. Serum IL-6 levels were elevated earlier than serum CRP or PSTI. There were significant relationships between the peak levels of serum IL-6 and the operation time (p less than 0.05) or the volume of blood loss during surgery (p less than 0.05), showing that serum IL-6 can be a useful laboratory test for the evaluation of tissue injuries due to surgical intervention. Though there was a significant relationship between the peak levels of IL-6 and CRP (p less than 0.01), there was no relationship between the peak levels of IL-6 and PSTI.