Our current knowledge about the bioavailability of provitamin A carotenoids in foods is insufficient, fragmentary and difficult to interpret. Past methods of estimating the vitamin A value of food carotenoids suffer both from uncertainty about the meaning of bioavailability and from the inadequacy of the indicators used in its determination. Reported conversion ratios of beta-carotene to vitamin A in humans in vivo, depending on conditions, range from 2:1 to 26:1 (microgram/microgram). Thus, the ratio of 6:1, devised by the World Health Organization, must be considered as a rough average estimate that is not applicable to all diets. Strategies to increase the dietary intake of carotenoid-containing foods should include measures to enhance carotenoid bioavailability.