Biochemical evidence of selenium (Se) deficiency is frequent in patients with chronic malnutrition. However, the incidence of Se deficiency in acutely ill patients is unknown. In 175 consecutive ICU patients, plasma Se measured during the first week of ICU admission was 0.66 +/- 0.21 mumol/L (mean +/- SD) and was less than that measured in 57 healthy blood donors (1.05 +/- 0.21 mumol/L, p less than .001). Sixty-eight percent of plasma Se concentrations fell below the lower limit of the reference range. Plasma Se decreased with the number of weeks in the ICU (r = .33, p less than .01) with values decreasing to 0.49 +/- 0.20 mumol/L during the fourth week. Urinary Se excretion measured in a subgroup of 20 patients was related to plasma Se concentration (r = .38, p less than .05), and inversely related to N balance (r = .50, p less than .01). We conclude that decreased plasma Se concentrations are common in ICU patients and that catabolic states are associated with increased Se losses. These losses are unlikely to account for the marked reductions in plasma Se concentrations, and the findings suggest there may be significant changes in the distribution of body Se during critical illness.