BACKGROUND This study investigated the relationship of serum homocysteine in early pregnancy with the risk of gestational hypertension (GH) and preeclampsia (PE) and with the severity of preeclampsia. MATERIAL AND METHODS In a retrospective cohort study, we identified 147 confirmed cases of preeclampsia (103 with mild PE and 44 with severe PE) and 147 confirmed cases of GH; 4418 women who remained normotensive and nonproteinuric throughout pregnancy served as controls. Maternal blood samples were collected at between 11 and 13 weeks of gestation to test serum concentrations of homocysteine (Hcy), folic acid, and VitB12. A logistic regression model was used to calculate adjusted odds ratios (aORs) and 95% confidence intervals (CIs). RESULTS Women who subsequently developed GH and PE were older and had higher body mass indexes (BMIs) than those in the control group. Compared with the control group, women who developed PE were less educated (P=0.031), and more of those who developed GH were primiparas (P=0.012). The serum levels of Hcy in severe PE were significantly higher than those in the control group (median: 8.50 μmol/L vs. 7.33 μmol/L, P<0.001). After logistic regression analyses for potential confounding factors, the adjusted odds ratios (aORs) of Hcy was 1.12 for severe PE (95% CI 1.06-1.20). The serum concentrations of folic acid and VitB12 in those with GH and PE were not significantly different from controls. CONCLUSIONS A high level of Hcy in the first trimester is an independent risk factor for severe PE, although it is not a useful marker for the subsequent development of GH and mild PE.