Selenium concentrations have been measured in plasma and in blood leucocytes from 29 haemodialysis patients and from 25 healthy men. The selenium contents of the plasma and white blood cells of the dialysis patients were significantly reduced (p less than 0.001). Selenium deficiency in humans results in a congestive cardiomyopathy and is associated with increased risks of accelerated atherosclerosis and cancer. Each of these is found with abnormal frequency in haemodialysis patients.