Vitamin B9 or folate is one of the vitamins essential for growth and development of the human organism. What we call folates are either natural compounds found in food or synthetic compounds such as folic acid. The difference between these two compounds is in the fact that synthetic compounds are more stable and have a greater bioavailability. Folates are necessary for the synthesis of purine and pyrimidine which make up deoxyribonucleic acid (DNA). They also have a role in amino acid metabolism, the most important amino acid being methionine. Sources of vitamin B9 are vegetables, especially green leafy vegetables, fruits, cereals and offal. Meat is not a good source of vitamin B9. Vitamin B9 deficiency can occur because of inadequate dietary intake of folate, increased metabolic need for folate or its malabsorption. Knowing that folic acid intake during the preconceptional or periconceptional period helps prevent neural tube defects, it is recommended that women in their generative age as well as pregnant women take 400 micrograms of folic acid per day. Besides preventing neural tube defects, vitamin B9 helps prevent anemia in pregnancy. Vitamin B9 deficiency can cause megaloblastic anemia, a methionine metabolism disorder which results in increased levels of homocysteine. Increased levels of homocysteine can damage vascular endothelial cells. That is why hyperhomocysteinemia is connected with the increased incidence of coronary disease, myocardial infarction, peripheral artery disease and stroke. One of the possible consequences of folate deficiency and an increased level of homocysteine is cognitive impairment. Depression and Alzheimer's disease are among the most common consequences of folate deficiency. Folate deficiency is also associated with the increased incidence of certain types of cancer such as colorectal cancer, prostate cancer, ovarian cancer, breast cancer, lung cancer, pancreatic cancer and melanoma. Since folate receptors are overexpressed on the surface of cancer cells, there are strategies for using folates in chemotherapy.