Toxicity has been associated with abuse of vitamin A supplements and with diets extremely high in preformed vitamin A. Consumption of 25,000-50,000 IU/d for periods of several months or more can produce multiple adverse effects. The lowest reported intakes causing toxicity have occurred in persons with liver function compromised by drugs, viral hepatitis, or protein-energy malnutrition. Certain drugs or other chemicals may markedly potentiate vitamin A toxicity in animals. Especially vulnerable groups include children, with adverse effects occurring with intakes as low as 1,500 IU.kg-1.d-1, and pregnant women, with birth defects being associated with maternal intakes as low as approximately 25,000 IU/d. The maternal dose threshold for birth defects cannot be identified from present data. An identifiable fraction of the population surveyed consumes vitamin A supplements at 25,000 IU/d and a few individuals consume much more. beta-Carotene is much less toxic than vitamin A.