Sex differences in endothelial cell (EC) biology may reflect intrinsic differences driven by chromosomes or sex steroid exposure and gender differences accumulated over life. We analysed EC gene expression data from boy-girl twins at birth and in non-twin adults to detect sex differences at different stages of life, and show that 14-25% of the EC transcriptome is sex-biased. By combining data from both stages of life, we identified sex differences that are present at birth and maintained throughout life, and those that are acquired over life. Promisingly, we found that genes that present with an acquired sex difference in ECs are more likely to be targets of sex steroids. Annotating both gene sets with data from multiple genome-wide association studies (GWAS) revealed that genes with an intrinsic sex difference in ECs are enriched for coronary artery disease GWAS hits. This study underscores the need for treating sex as a biological variable. Abbreviations CAD Coronary artery disease DEG Differentially expressed gene EC Endothelial cell HAEC Human aortic endothelial cell HUVEC Human umbilical vein endothelial cell GWAS Genome-wide association study Endothelial cells (ECs) are located at the innermost lining of all vessels and are responsible for circulatory homeostasis. ECs are implicated in a plethora of functions, such as water and nutrient transfer, inflammation, vascular tension, hemostasis, and angiogenesis 1. Differences in EC biology between the sexes have been described, indicating that sex is an important variable in EC biology 2. Studies focussing on functional differences, i.e. migration, proliferation, and shear stress response, imply sex differences in regulation of key endothelial pathways, such as the redox system 3-5. Differences between female and male cells might be present from birth onwards, or acquired later in life because of sex steroid exposure and gender differences. Pinpointing master regulators herein has been difficult, as differences between the sexes may be caused by sex hormones and/or sex chromosomes, which are difficult to study separately or to dissect. Transcriptomic studies of female and male (endothelial) cells shed light on differentially expressed genes (DEGs) between the sexes and their regulators, but few have been performed thus far on ECs 4. Tissue-specific comparisons have been made using publicly available data from the