Preeclampsia (PE) is a disorder specific to pregnancy characterized by new-onset hypertension and proteinuria after 20 weeks of gestation. There is no definite treatment for PE except delivery of the placenta. The purpose of this study was to elucidate the biological pathways involved in the development of PE and to discover a novel biomarker for PE by performing global gene expression analysis of amniotic fluid cell-free RNA.The participants were recruited from the Department of Obstetrics and Gynecology of CHA Gangnam Medical Center (Seoul, Korea) between March 2014 and February 2015. Eight samples were collected from 8 subjects at second trimester who were later diagnosed with PE. From the amniotic fluid samples, cell-free RNA extraction was performed and gene expression was analyzed using the GeneChip PrimeView Array. Transcriptome data previously analyzed by our group from 9 euploid mid-trimester amniotic fluid samples were used as the control for comparative analysis. Functional analysis of the probe sets was performed using the online Database for Annotation, Visualization, and Integrated Discovery (DAVID) toolkit 6.7.We identified 1841 differentially expressed genes (DEGs) between the PE group and the control. Of these, 1557 genes were upregulated in the PE group, while 284 genes were upregulated in the control. The functional annotation of DEGs identified specific enriched functions such as "transport," "signal transduction," and "stress response." Functional annotation clustering with enriched genes in the PE group revealed that translation-related genes, cell-cell adhesion genes, and immune-related genes were enriched. KEGG pathway analysis showed that several biological pathways, including the ribosome pathway and various immune pathways, were dysregulated. Several genes, including RPS29, IGF-2, and UBC, were significantly upregulated in PE, up to tenfold.This study provides the first genome-wide expression analysis of amniotic fluid cell-free RNA in PE. The results showed that gene expression involving the ribosome pathway and immunologic pathways are dysregulated in PE. Our results will aid in understanding the underlying pathogenesis of PE.