The selenite anion is a selenium oxoanion with the chemical formula SeO2−3. A selenite (Se) is a compound that contains this ion. Sodium selenite is an inorganic form of the trace element selenium with potential antineoplastic activity. It was documented that Se deficiency observed in some countries and/or geographic regions (e.g. Keshan region in China), is associated with an increased morbidity and mortality of neoplastic diseases. To correct this problem a number of organic and inorganic selenium compounds were developed and tested. However, it is now firmly established that only an inorganic sodium selenite with four-valent Se, and not that with six-valent (selenate) cation shows anticancer activity. Selenite can undergo redox reaction, for example with protein's sulfhydryl groups expressed on the surface of tumor cells. In this way selenite prevents non-enzymatic formation of parafibrin that coats tumors cells and hence presents them as 'self' to the innate cellular immune system. Consequently, macrophages of the lymphatic system do not recognize neoplastic cells as 'foreign' bodies and spare them from the immune destruction. Sodium selenite also showed promise as a cost-effective, nontoxic anti-inflammatory agent. Treatment with sodium selenite lowers reactive oxygen species (ROS) production, causes a spontaneous reduction in lymphedema volume, increases the efficacy of physical therapy for lymphedema, and reduces the incidence of erysipelas infections in patients with chronic lymphedema. Limited evidence has been presented though that intakes of selenium greater than the amount needed to allow full expression of selenoproteins may have chemopreventive effects against cancer. Controlled intervention studies are needed to fully evaluate selenium as a cancer chemopreventive agent. The US Food and Drug Administration approved a selenium supplement to animal diets; the most common form is sodium selenite for pet foods.