The objective of this study was to quantify the mediating role of inflammation and triglycerides in the association between prepregnancy body mass index (weight (kg)/height (m)2) and preeclampsia. The authors conducted a nested case-control study of 55 preeclamptic women and 165 pregnant controls from the Pregnancy Exposures and Preeclampsia Prevention Study (Pittsburgh, Pennsylvania, 1997-2001). Serum samples collected at < or = 20 weeks' gestation were analyzed for levels of C-reactive protein and triglycerides. The adjusted odds ratio (AOR) from a multivariable conditional logistic regression model assessing the total effect of body mass index on preeclampsia risk was compared with the AOR from the same model after results were controlled for C-reactive protein, triglycerides, and confounding factors (direct-effects model). The percentage of the total effect that was mediated through inflammation and triglycerides was calculated as 100 - [ln(direct-effects AOR)/ln(total-effects AOR)]. In the total-effects model, 4- and 8-unit increases in body mass index were associated with 1.7-fold (95% confidence interval (CI): 1.3, 2.3) and 2.9-fold (95% CI: 1.6, 5.2) increases in preeclampsia risk, whereas in the direct-effects model, these AORs were 1.4 (95% CI: 1.0, 1.9) and 2.0 (95% CI: 1.0, 3.8), respectively. Inflammation was a more important mediator than triglycerides. These findings suggest that approximately one third of the total effect of body mass index on preeclampsia risk is mediated through inflammation and triglyceride levels.