Technical Abstract: Although selenium (Se) is essential for sperm function experimental animals, high Se intake in humans has been associated with both improvements and impairments in semen quality. We previously reported a decrease in sperm motility in five men fed high-Se foods in a metabolic research unit, but could not rule out other environmental and dietary factors. To follow up these observations, we conducted a randomized, placebo-controlled trial of Se supplementation in 42 free-living men. Subjects were administered 300 'g Se/d as high-Se Baker¿s yeast, or low-Se placebo yeast for 48 weeks. Semen analysis was performed four time during the three-week run-in period and then twice a week at 6, 12, 24, 36, 48, 72 and 96 weeks. Blood samples were collected three times during the run-in period and once at each subsequent visit. Se concentration increased 50% in blood plasma and 40% in seminal plasma. Se supplementation did not affect serum androgen concentrations, sperm Se concentration or sperm count, motility, progressive velocity, or morphology. However, serum LH, semen volume and sperm Se decreased in both groups throughout the supplementation and follow-up periods. At the same time, sperm straight line velocity and percent normal morphology increased in both groups. Even though the subject¿s dietary Se intake was tripled and their total body Se was doubled by 48 weeks of supplementation, their testicular Se stores were unaffected. These results are consistent with animal studies showing the Se status of testes to be remarkably refractory to dietary Se intake.