Selenium is an essential trace element in nutrition for the prevention of disease in humans. Epidemiological studies indicate an association between low nutritional selenium status and increased risks of cardiomyopathy, cardiovascular disease, and carcinogenesis in various sites of the body. The role of selenium supplementation in the prevention and treatment of AIDS-related pathology has been considered. Selenoproteins discovered in mammalian cells may account for the essentiality of selenium in the body's antioxidant defense; thyroid hormone function; immune system function, particularly the cellular immunity; formation of sperm; and functioning of the prostate gland. The seleno-organic compounds, primarily L-(+)-selenomethionine, generally are recognized as safe and effective forms of selenium supplementation. The nutritionally recommended dose of elemental selenium is estimated at 50 to 200 micrograms [corrected] per day. There is, however, increased discussion of a pharmacological dose of selenium, significantly higher than the nutritional dose of the microelement, to treat active conditions. One way of increasing the tissue levels of selenium is to combine its ingestible form with a nutrient bioavailability enhancing compound.