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To determine the efficacy of high-dose folic acid for the prevention of preeclampsia in twin pregnancies. Secondary analysis of a randomized controlled trial in 70 obstetrical sites in Argentina, Australia, Canada, Jamaica, and the UK between 2011 and 2015. Eligible women pregnant with twins who were aged 18 y or older and between 8 and 16 completed weeks' gestation were randomized between to receive daily high-dose folic acid (4.0–5.1 mg) or placebo. The primary outcome was preeclampsia, presenting as hypertension after 20 weeks' gestation with significant proteinuria. Secondary outcomes included severe preeclampsia, preterm birth, and adverse fetal and neonatal outcomes. Of 2464 participants randomized between 18 April 2011 and 14 December 2015, 462 (18.8%) had a confirmed twin pregnancy. Thirty-four of these participants withdrew consent or did not have primary outcome data available, and 428 women were analyzed. The rate of preeclampsia was significantly higher in the folic acid group compared to the placebo group in crude analyses (17.2 versus 9.9%; relative risk 1.75 [95% CI 1.06–2.88], p =.029). Multivariable analyses attenuated this effect, rendering it not statistically significant (RR 1.58 [95% CI 0.95–2.63], p =.079). High-dose folic acid supplementation was not significantly associated with preeclampsia in a subgroup of twin pregnancies. Although a suggested elevated risk cannot be confirmed, these results may help to gain novel insights in the etiology of preeclampsia, which continues to be poorly understood. ClinicalTrials.gov NCT01355159. [ABSTRACT FROM AUTHOR]