Selenium is a trace element essential to human health. Critical illness is associated with the generation of oxygen free radicals resulting in a condition of oxidative stress. Supplementing critically ill patients with antioxidant nutrients may improve survival. Selenium levels can be low due to redistribution to high-priority organs and dilution associated with aggressive resuscitation of the patient. The purpose of this review is to investigate the benefit of selenium supplementation in critically ill patients.Most of the selenium-supplementation trials were performed in relatively small patient populations presenting with trauma, sepsis, burns and adult respiratory distress syndrome. Widely varying doses of selenium of between 200 and 1000 microg were used, either alone or in combination with other antioxidants. Significant improvements have been demonstrated in length of hospital stay, rate of infection and need for haemodialysis in these patients. However, no trial has demonstrated a statistically significant improvement in mortality. Two recent meta-analyses suggest a trend towards reduced mortality with selenium supplementation.Selenium, by supporting antioxidant function, may be associated with a reduction in mortality. To demonstrate this large, well-designed randomized trials are required.