The notion that requirements for folic acid may be higher in some tissues than others, resulting in localized deficiencies in spite of blood levels in the normal range was first suggested by the observation of megaloblastic changes in the cervical epithelium that responded to folate supplementation. Theoretically, such deficiencies may arise from elevated folate turnover in response to rapid tissue proliferation or repair; inactivation or alteration of its function by external agents such as tobacco, alcohol, or drugs; or altered metabolism or tissue uptake caused by an inborn error. Marginal dietary intake could aggravate these effects on cells at risk. Evidence for the possible existence of localized folate deficiencies in the aerodigestive tract includes lower circulating folate levels in smokers as compared with nonsmokers; yet lower circulating levels in smokers with bronchial metaplasia; lower folate levels in scrapings of the buccal mucosa of smokers than non-smokers; apparent improvement in bronchial atypical metaplasia in smokers supplemented with folic acid; lower erythrocyte folate levels and higher prevalence of cellular features compatible with folate deficiency in geographic areas and individuals in South Africa at high risk for esophageal cancer; and a trend toward a lower prevalence of colonic dysplasia in ulcerative colitis patients who use folic acid supplements. These observations, as well as animal and in vitro studies, also suggest that folate deficiency may be co-carcinogenic. Further research in this area will be aided by the development of animal models of localized folate deficiency and of methodologies capable of measuring folate levels in minute quantities of tissues and exfoliated cells.