We have developed a high sensitive method of measuring the serum endocannabinoids (anandamide, ABA and 2-arachidonyl-glycerol, 2-AG) by gas chromatography and mass spectrometry (GC/MS) with isotope dilution. We measured the serum endocannabinoids in 5 esophageal and 5 lung cancer patients perioperatively with GC/MS to clarify the relationship between perioperative inflammation and endocannabinoids. It was demonstrated that ABA didn' t significantly change perioperatively, although the pre and postoperative 2-AG concentrations in the cancer patients were significantly higher than the control value (P<0.05) and that AEA did not change perioperatively in significant manner. Therefore, 2-AG may play a pivotal role in perioperative inflammation.