Folic acid is important because supplementation around the time of conception has been proven to lower the risk of having offspring with a neural-tube defect. Furthermore, both dietary folate and folic acid decrease plasma total homocysteine concentrations. Elevated plasma homocysteine concentrations are considered to be an independent risk factor for cardiovascular disease. The aim of the present review is to give an overview of factors influencing bioavailability and bioefficacy (the proportion of ingested nutrient converted to its active form) of food folate and folic acid, and to discuss the functional bioefficacy of folate and folic acid in decreasing plasma homocysteine concentrations. We use the mnemonic SLAMENGHI to group factors influencing bioavailability and bioefficacy: Species of folate; Linkage at molecular level; Amount of folate and folic acid consumed; Matrix; Effect modifiers; Nutrient status; Genetic factors; Host-related factors; mathematical Interactions between the various factors. Bioefficacy of folate from some foods is 50 % that of folic acid. This factor is most probably explained by the matrix factors, encapsulation and binding. However, often such effects cannot be distinguished from factors such as species, chain length of folate in food, effect modifiers and the amount of folate consumed in a meal. Folic acid provided as a supplement is well absorbed. However, the homocysteine-lowering capacity of doses of folic acid >500 mug is limited. It is unclear whether unmetabolised folic acid poses health risks. This factor is of importance, because food fortification is now implemented in many countries and folic acid supplements are freely available. In particular circumstances host-related factors, such as gastrointestinal illness and pH of the jejunum, can influence bioavailability. Genetic factors also deserve attention for future research, because polymorphisms may influence folate bioavailability.