Selenium is an essential trace element important for glutathione peroxidase activity. Selenium deficiency has been found in association with skeletal and cardiac myopathy and may increase the risk for cardiovascular diseases and for cancer. We studied 39 hemodialysis patients and 15 control subjects. Plasma selenium, plasma glutathione peroxidase activity and erythrocyte glutathione peroxidase activity were lower than in controls (38 +/- 14 vs. 88 +/- 17 micrograms/liter (P less than 0.01); 153 +/- 32 vs. 334 +/- 41 IU/liter (P less than 0.01), 19 +/- 4 vs. 26 +/- 4 IU/g Hb (P less than 0.01), respectively). Plasma selenium and plasma glutathione peroxidase activity were strongly correlated with duration of dialysis. There was no correlation between plasma selenium and protein or calorie intakes. Plasma selenium was lower in patients dialyzed with highly permeable membranes (P less than 0.01). The total muscle mass, assessed by anthropometry, was lower in the patients who had the lowest plasma selenium (P less than 0.01) and plasma glutathione peroxidase activity (P less than 0.05). Interventricular septum hypertrophy, documented by echocardiography, was greater in patients with the lowest plasma selenium and plasma glutathione peroxidase activity (P less than 0.01). Twenty hemodialysis patients had oral supplementation of 500 micrograms/day of sodium selenite for three months, and then, 200 micrograms/day for the next three months. Plasma selenium increased as early as the first week and reached a plateau similar to the control levels after three weeks. Plasma glutathione peroxidase activity increased after two months but remained below controls. Erythrocyte glutathione peroxidase activity reached a higher value than controls after one month.(ABSTRACT TRUNCATED AT 250 WORDS)