To compare indicators of systemic inflammatory response in the second trimester in women who developed pre-eclampsia with normal pregnancies. Prospective nested case control study derived from a cohort of 2190 pregnant women. Blood samples were obtained at 18 weeks of gestation. The following inflammatory parameters were measured: tumour necrosis factor-alpha (TNF-alpha), plasminogen activator inhibitor-1 (PAI-1), interleukin-1beta (IL-1beta), IL-6, IL-10, microCRP and tissue factor (TF). Institute of Medical Genetics, University of Oslo, and Department of Medical Genetics, Ullevål University Hospital and Departments of Obstetrics and Gynecology, Aker University Hospital, Oslo, Norway. The cases were 71 women who subsequently developed pre-eclampsia. The controls were 71 healthy, pregnant women matched for age, parity and first trimester body mass index (BMI). Venous blood was drawn from fasting subjects into 5 mL test tubes containing EDTA. Samples were analysed for inflammatory parameters: IL-1-beta, IL-6, IL-10, TNF-alpha, PAI-1, TF (ELISA-technique) and CRP (latex-enhanced immunonephelometric assay), strictly according to the manufacturer's recommendation. The matched case and control subjects were compared by the paired two-tailed Wilcoxon signed rank test. All P values were two-tailed and P < 0.05 was deemed statistically significant. We found no differences in plasma concentrations of PAI-1, IL-1beta, IL-6,IL-10, microCRP, TNF-alpha or TF at 18 weeks of gestation between women who subsequently developed pre-eclampsia and matched control women. In contrast to findings from women with overt pre-eclampsia, the present study indicates that there are no indications of intensified systemic inflammatory response at 18 weeks of gestation in women who later develop pre-eclampsia.