['To determine the relation between plasma redox status and severity of illness for patients admitted to an intensive care unit (ICU).', 'A prospective cohort study.', 'A mixed medical and surgical adult ICU with 12 beds.', 'A total of 73 consecutive patients admitted to the ICU.', 'Venous blood samples were routinely obtained within 24 hrs of admission.', 'Plasma total antioxidant capacity and lipoperoxides were measured by spectrophotometric technique at admission to the ICU. The plasma ratio total antioxidant capacity (mM)/lipoperoxides (microM) was used as an index of plasma redox status. Plasma concentration of the markers of leukocyte activation myeloperoxidase (enzyme-linked immunosorbent assay) and polymorphonuclear-elastase (immunoactivation assay) were also measured at admission to the ICU. Analysis of correlation between plasma ratio total antioxidant capacity/lipoperoxides and APACHE III score showed a negative association (p < .001, Spearman correlation test). Myeloperoxidase and polymorphonuclear-elastase correlated positively with Acute Physiology and Chronic Health Evaluation III scores (r2 = 0.58; p < .001; and r2 = 0.05; p = .035; respectively).'] Does plasma redox status relate to severity in critically ill patients?