OBJECTIVE Previous clinical and experimental studies have shown evidence of a leakage of whole bacteria and bacterial products after major trauma through the gut barrier. By determining plasma endotoxin levels, products of the arachidonic pathway, interleukin-6, and the endotoxin-neutralizing capacity (ENC) of plasma during colonoscopy, we studied the gut barrier function and the pathogenetic sequelae of mediator release during a minimally invasive procedure. METHODS Thirty-two patients were enrolled in a controlled prospective study. Endotoxin and ENC were determined by a chromogenic modification of the limulus amebocyte lysate test. Prostanoids and interleukin-6 were measured using commercially available ELISA tests. C-reactive protein levels were checked by nephelometry. RESULTS Twenty-one of the 32 patients had elevated endotoxin plasma levels during colonoscopy. In one patient, gut-derived bacteria were detected in plasma. ENC decreased after 5 min, and thromboxane B2 levels also started to increase at that time. No acute-phase response took place after 24 h. CONCLUSIONS During colonoscopy, endotoxin can be detected in blood. ENC measurement was shown to be even more sensitive. The pathogenetic sequelae leading to gut barrier failure remain unclear, because mediator release and endotoxemia, as checked by ENC, took place simultaneously.