The authors measured maternal third trimester plasma folate, vitamin B12, and homocyst(e)ine concentrations among 125 women with preeclampsia and 179 normotensive women in Lima, Peru (1997-1998), to determine whether these analytes were associated with the occurrence of preeclampsia. Logistic regression procedures were used to calculate maximum likelihood estimates of odds ratios and 95% confidence intervals. Relative to women in the upper quartile of the control distribution of maternal plasma folate concentrations, women with values in the lowest quartile experienced a 1.6-fold increased risk of preeclampsia (odds ratio = 1.6; 95% confidence interval: 0.8, 3.2). There was no evidence of an increased risk of preeclampsia associated with low plasma vitamin B12 concentrations. The unadjusted relative risk of preeclampsia increased across successively higher quartiles of plasma homocyst(e)ine level (odds ratios were 1.0, 1.0, 1.5, and 2.9, respectively, with the lowest quartile used as the referent; p for linear trend = 0.0004). After adjustment for maternal age, parity, gestational age, use of prenatal vitamins, whether the pregnancy had been planned, and educational attainment, the relative risk between extreme quartiles was 4.0 (95% confidence interval: 1.8, 8.9). These findings are consistent with earlier reports suggesting that hyperhomocyst(e)inemia in pregnancy may be a risk factor for preeclampsia.