This study was undertaken to address the role of oxidative stress in preeclampsia.We measured urinary 8,12-iso-iPF(2alpha)-VI, a chemically stable, free-radical catalyzed product, in a case control study of severe preeclampsia nested within the trial of Calcium for Preeclampsia Prevention. Cases included 29 women who developed severe preeclampsia and from whom urine had been obtained 10 to 20 weeks before the diagnosis of preeclampsia, 3 to 9 weeks before, and 1 day before through delivery. Controls did not develop hypertension or proteinuria and were matched to cases by center, gestational age at each of 3 corresponding urine collections, and date of enrollment.Urinary 8,12-iso -iPF(2alpha)-VI did not differ significantly between cases and controls before or at diagnosis of preeclampsia, nor did it vary with gestational age.These results call into question the importance of oxidative stress in the disease and the biochemical rationale for clinical trials of antioxidants to prevent and treat preeclampsia.