In order to assess if an oxidant/antioxidant imbalance is involved in human septic shock and its outcome, we measured plasma levels of the lipid peroxides malondialdehyde--as thiobarbituric acid reactive substance--conjugated dienes and fluorescent products, together with the antioxidants alpha-tocopherol, glutathione peroxidase activity and selenium in 12 patients with septic shock and compared them with values of normal controls. At first measurements, malondialdehyde (median 3.9 mumol/l; range 2-38.8) and fluorescent products (median 21.2%; range 9.4-134) were elevated (p less than 0.05), alpha-tocopherol (median 15 mumol/l; range 7-25) and selenium (median 0.76 micrograms/ml; range 0.49-1.09) were depressed (p less than 0.05). Conjugated dienes and glutathione peroxidase activity were in the normal range. In non-survivors (n = 5) initial levels of malondialdehyde and fluorescent products (median 11 versus 3.1 mumol/l; 74 versus 13% respectively) were higher than in survivors (p less than 0.05) and initial selenium levels were lower (median 0.58 versus 0.92 micrograms/l; p less than 0.05). These results are consistent with the concept that an oxidant/antioxidant imbalance--as indicated by elevated plasma lipid peroxides and depressed antioxidants--is involved in human septic shock and a fatal outcome.