Background: Electronic cigarettes (e-cigarettes) have experienced a tremendous increase in use. Unlike cigarette smoking, the effects of e-cigarettes and their constituents on mediating vascular health remain understudied. However, given their increasing popularity, it is imperative to evaluate the health risks of e-cigarettes, including the effects of their ingredients, especially nicotine and flavorings. Objectives: To investigate the effects of flavored e-cigarette liquids (e-liquids) and serum isolated from e-cigarette users on endothelial health and endothelial cell dependent macrophage activation. Methods: We used human induced pluripotent stem cell-derived endothelial cells (iPSC-ECs) and a high-throughput screening approach to assess endothelial integrity following exposure to 6 different e-liquids with varying nicotine concentrations and to serum from e-cigarette users. Results: The cytotoxicity of the e-liquids varied considerably, with the cinnamon-flavored product being most potent and leading to significantly decreased cell viability, increased reactive oxygen species (ROS) levels, caspase 3/7 activity, and low-density lipoprotein uptake, activation of oxidative stress-related pathway, and impaired tube formation and migration, confirming endothelial dysfunction. Upon exposure of ECs to e-liquid, conditioned media induced macrophage polarization into a pro-inflammatory state, eliciting the production of interleukin-1β (IL-1β) and IL-6, leading to increased ROS. After exposure of iPSC-ECs to serum of e-cigarette users, we observed increased ROS linked to endothelial dysfunction, as indicated by impaired pro-angiogenic properties. We also noted an increase in inflammatory cytokine expression in serum of e-cigarette users. Conclusions: Acute exposure to flavored e-liquids or e-cigarette use exacerbates endothelial dysfunction, which often precedes cardiovascular diseases.Abbreviations CRP = c-reactive protein CVD = cardiovascular disease E-cigarette = electronic cigarette E-liquids = electronic cigarette liquids iPSC-ECs = induced pluripotent stem cell-derived endothelial cells PG = propylene glycol PLT = platelet ROS = reactive oxygen species VG = vegetable glycerin Condensed Abstract E-cigarettes have seen a rapid increase in use although their effects on vascular health remain understudied. Here, we investigated the effects of flavored e-cigarette liquids (e-liquids) on endothelial health by exposing human induced pluripotent stem cell-derived endothelial cells (iPSC-ECs) to different e-liquids with varying nicotine concentrations. While cytotoxicity varied among the tested flavors, the cinnamon-flavored product was most potent leading to decreased cell viability, increased oxidative stress and caspase activity, and impaired tube formation confirming endothelial dysfunction; results which were further corroborated using e-cigarette