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Background: Age-related hearing loss is a common chronic condition of elderly persons. Low folate status has been associated with poor hearing. Objective: To determine whether folic acid supplementation slows age-related hearing loss. Design: Double-blind, randomized, placebo-controlled trial conducted from September 2000 to December 2004. Setting: The Netherlands. Participants: 728 older men and women recruited from municipal and blood bank registries with plasma total homocysteine concentrations 13 mu mol/L or greater serum and vitamin B-12 concentrations 200 pmol/L or greater at screening, and no middle ear dysfunction, unilateral hearing loss, or pathologic ear conditions unrelated to aging. Intervention: Daily oral folic acid (800 mu g) or placebo supplementation for 3 years. Measurements: 3-year change in hearing thresholds, assessed as the average of the pure-tone air conduction thresholds of both ears of the low (0.5-kHz, 1-kHz, and 2-kHz) and high (4-kHz, 6-kHz, and 8-kHz) frequencies. Results: Initial median hearing thresholds were 11.7 dB (interquartile range, 7.5 to 17.5 dB) for low frequencies and 34.2 dB (interquartile range, 22.5 to 50.0 dB) for high frequencies. Sixteen participants (2%) were lost to follow-up. After 3 years, thresholds of the low frequencies increased by 1.0 dB (95% CI, 0.6 to 1.4 dB) in the folic acid group and by 1.7 dB (CI, 1.3 to 2.1 dB) in the placebo group (difference, -0.7 dB [CI, -1.2 to -0.1 dB]; P = 0.020). Folic acid supplementation did not affect the decline in hearing high frequencies. Limitations: The strict criterion for participation on the basis of serum homocysteine concentrations limits extrapolation to the general population. Folic acid fortification of food was prohibited in the Netherlands during the study, so baseline folate levels in participants were about half of those found in the U. S. population. Conclusions: Folic acid supplementation slowed the decline in hearing of the speech frequencies associated with aging in a population from a country without folic acid fortification of food. The effect requires confirmation, especially in populations from countries with folic acid fortification programs.