Objective: We recently developed a novel sandwich chemiluminescence assay to determine serum concentrations of periostin. Periostin has high amino acid homology with transforming growth factor-β–induced protein βigh3, a molecule induced by transforming growth factor-β₁, which promotes the adhesion and spreading of fibroblasts. It is also homologous with the insect cell adhesion molecule fasciclin I. We determined serum periostin concentrations in women with preeclampsia and in normotensive pregnant women. Study Design: The study groups included 30 women with preeclampsia and 30 normotensive pregnant women at Magee-Womens Hospital. Blood samples collected from these women were assayed for periostin by use of newly developed monoclonal and polyclonal antibodies. We studied periostin messenger ribonucleic acid (mRNA) expression in human tissues by using reverse transcriptase–polymerase chain reaction and in situ hybridization. Results: Serum periostin concentrations were elevated in patients with preeclampsia (mean ± SD, 311.8 ± 56.3 ng/mL) compared with normotensive pregnant women (218.8 ± 37.3 ng/mL). No correlation was found between serum concentrations of periostin and concentrations of TGF-β₁, vascular cell adhesion molecule-1 (VCAM-1), E-selectin, and interleukin-6. Reverse transcriptase–polymerase chain reaction analysis demonstrated human periostin expression in lung, kidney, and placenta but not in the heart, liver, brain, or skeletal muscle. For periostin complementary deoxyribonucleic acid cloned from placenta, there was a splicing deletion within the C-terminal domain. In situ hybridization data showed that the periostin gene was expressed in the stroma cells of placenta. Conclusion: Our study suggests that human periostin may play a role in the pathogenesis of preeclampsia. Although its function remains unclear, the expression of periostin as an adhesion molecule could suggest novel mechanisms in preeclampsia. (Am J Obstet Gynecol 2002;186:103-8.)