Objective. To determine whether urinary 8-hydroxy-2?-deoxyguanosine (8-OHdG) levels are altered in patients with septic shock and whether polymyxin B-immobilized fibre (PMX-F) haemoperfusion reduces the 8-OHdG level in these patients. Design and subjects. Twenty patients with septic shock and 20 age-matched healthy volunteers were included in the study. The septic shock patients were divided into two treatment groups: a PMX-F treatment group (n 012) and a non-PMX-F treatment group (n08). Standard supportive care was continued without change during PMX-F haemoperfusion. Clinical markers, including plasma endotoxin, were measured before and after the first and the second PMX-F treatment and the following day. Urinary 8-OHdG levels were also examined before and after the first and the second PMX-F treatment (24 h pooled urine) and the following day. Results. Urinary 8-OHdG levels were significantly higher in septic shock patients (median 38.0 ng/mg creatinine; range 16.0Á52.0 ng/mg creatinine) than in healthy volunteers (5.5 ng/mg creatinine; range 4.5Á7.5 ng/mg creatinine) (pB0.01). Urinary 8-OHdG levels correlated significantly with plasma endotoxin levels (pB 0.01), the Acute Physiology and Chronic Health Evaluation score (pB0.01) and the Sepsis-related Organ Failure Assessment score (pB0.01). PMX-F haemoperfusion reduced the plasma endotoxin and urinary 8-OHdG levels significantly after the first (p B0.01) and the second treatment (pB0.001) and the following day (pB0.001). However, these markers did not change significantly with non-PMX treatment. Conclusions. An increased urinary 8-OHdG level appears to be associated with septic shock, and PMX-F haemoperfusion is effective in reducing urinary 8-OHdG levels in patients with septic shock.