The present experiment was undertaken to determine the effects of dietary supplements of folic acid administered from 4 wk prepartum to 305 d of lactation on lactational performance. Sixty-three Holstein cows were assigned to 22 blocks of 3 cows according to lactation number, milk production, and body weight (BW). Within each block, cows received 0, 2, or 4 mg of folic acid/kg of BW per d. Dietary supplements of folic acid increased serum and milk folates but affected milk production and composition of primiparous and multiparous cows differently. Supplementary folic acid had little effect on milk production and composition of primiparous cows, except that milk production decreased during the first 100 d of lactation. However, during a complete lactation (3 to 305 d after calving), supplementary folic acid was associated with increased milk production by multiparous cows (8284 +/- 560, 8548 +/- 380, and 8953 +/- 191 kg for cows fed diets supplemented with 0, 2, and 4 mg of folic acid/kg of BW per d, respectively). The percentage of ash in milk was decreased for cows fed the highest amount of dietary folic acid. During the first 100 d of lactation, supplementary folic acid was associated with a lower concentration of nonprotein nitrogen in the milk of multiparous cows. The present study confirms results obtained previously, suggesting that, although the supply of folates from an unsupplemented diet and the ruminal microflora is sufficient to avoid a deficiency in folic acid, supplementary folic acid may increase the milk production of cows in the second lactation or greater.