To evaluate the efficacy of sodium selenite in Keshan disease, a study was carried out among children in Mianning county, Sichuan province, from 1974 to 1977 inclusive. The morbidity of Keshan disease in Selenium (Se) supplemented children was found to be 2.2‰ and 1.0‰ in 1974 and 1975 respectively while that in the control groups was 13.5‰ and 9.5‰, the difference being highly significant in both years. In 1976, both groups were given sodium selenite and the rate was further decreased to 0.32‰. In 1977, no new cases of Keshan disease were discovered among the treated children. Of 36,603 Se supplemented children, 21 cases of the disease occurred during the 4 years of investigation. Of these, only 3 died and 1 became a chronic. Among 9,642 children in the control group, 107 cases occurred, of which 53 became fatal and 6 had insufficient heart function. These results indicate that sodium selenite could not only lower the morbidity of Keshan disease but also alleviate its clinical signs and improve its prognosis. The role of selenium in the etiology of Keshan disease and the possible mechanism of sodium selenite in its prevention are discussed.