Objective: This case-control study was conducted in Lima, Peru, from June 1997 through January 1998 to assess whether alteration in maternal erythrocyte omega-3 (n-3) and omega-6 (n-6) fatty acids was associated with increased risk of preeclampsia. Methods: A total of 99 preeclampsia and 100 normotensive pregnant women were included. Maternal erythrocyte n-3 and n-6 fatty acids were determined using capillary gas chromatography/mass spectrometry and expressed as micromolar (mM) concentrations. We employed logistic regression procedures to estimate odds ratios (ORs) and 95% confidence intervals (CIs). Result: n-3 fatty acids were consistently lower in preeclampsia cases than controls. After adjusting for confounders, the corresponding ORs for preeclampsia across decreasing quartiles of sum of long-chain n-3 fatty acids were 1.0, 3.3, 2.4, and 3.3, respectively (P = 0.07 for trend). A similar pattern was observed for eicosapentenoic acid (20:5n-3, EPA) and docosahexenoic acid (22:6n-3, DHA). There was no clear evidence of an association between arachidonic acid (20:4n-6, AA) and preeclampsia risk, the ORs in successively lower quartiles were 1.0, 1.1, 1.0, and 1.5 (P = 0.48 for trend). A similar pattern was seen for the sum of long-chain n-6 fatty acids. Conclusion: In Peruvian women, low erythrocyte n-3 fatty acids appeared to be associated with an increased risk of preeclampsia.