In a randomized, placebo controlled, double-blind Phase III clinical trial, selenium supplements were given to patients in 14 villages in Chandpur District in Bangladesh to combat chronic arsenic poisoning resulting from drinking arsenic laced groundwater. About 413 subjects were given 200 μg sodium selenite per day as oral supplements and 408 subjects received matching placebo. Urine Arsenic (As), Selenium (Se) and Plasma Selenium (Se) were analyzed at 0, 24 and 48 weeks of supplementation. In addition, Selenoprotein Glutathione Peroxidase (GPx), Thioredoxin Reductase (TrxR) and selenoprotein P were measured in select few plasma samples at 0, 24 and 48 weeks following selenite supplementation. The trial was successfully conducted with high compliance as measured from DOTS and correct treatment assignment and dispensation as evidenced by the Se levels in plasma and urine. No consistent effect of selenite supplementation on plasma and urinary arsenic levels were observed. However, Glutathione Perxidase (GPx), Thioredoxin Reductase (TrxR) activity and selenoprotein P in plasma seem to increase with supplementation. Selenium supplementation appears to increase plasma selenium and selenoprotein levels in arsenicosis patients.