Alkylating agents such as sulfur mustard (S-Lost, mustard gas, SM, HD, bis(2- chloroethyl)sulfide, Yperite), predominantly used by military forces, and related chemotherapeutic drugs, e.g. chlorambucil (Cbl, trade name: Leukeran), are known to cause severe cellular damage without being fully understood on a molecular level. The discovery of sulfur mustard and its use as a chemical weapon was followed by the detection of associated, medically useful toxicological properties, which paved the way for the development of alkylating chemotherapeutics. Used since the beginning of the 20th century, SM has been deployed in several conflicts around the world. It can lead to severe tissue damage - in particular dermal, mucosal, ocular and respiratory lesions - followed by impaired wound healing. While investigations of the acute toxicity have been the main focus up to now, explanations of long-term effects remain widely obscure. Increased understanding of the molecular mechanisms involved might help to identify new potential targets that could improve the general therapy of victims and the development of specific therapeutics. The ability of SM to cause lasting harm (e.g. cancer) and degenerative tissue damage after even a single exposure might - among other pathological processes - be explained by the involvement of underlying epigenetic modulations. The scientific field of epigenetics comprises alterations and modifications in gene expression that are not structurally changing the DNA sequence itself, but affecting chromatin organization as well as maintenance by e.g. DNA methylation and modifying histone patterns. SM is mainly used as a liquid or aerosolic chemical weapon agent (CWA) when dispersed, and directly interacts with tissues it comes into contact with first. As well as the aforementioned tissues, this interaction also involves corresponding small blood vessels. Blood vessel malformation (e.g. cherry hemangioma) is a frequent observation after SM exposure. Blood vessel formation depends on regeneration and immigration of endothelial cells. Early endothelial cells (EEC), which are known to play an important role in the formation of granulation tissue and the process of wound healing, provide a rational in vitro model to analyze the molecular toxicology of SM. After determining the lethal concentrations of sulfur mustard and assessing specific doses (0.5 μM, 1.0 μM) at which EECs are affected but maintain their cell division and proliferation abilities, we analyzed selected epigenetic modulators; a potential up- and downregulation of epigenetically relevant genes was examined. The EECs were tested for histone di- methylation (H3-K9, H3-K27, H3-K36), histone acetylation (H3-K9, H3-K27, H4-K8) and global DNA methylation (5-mc, 5-hmc). The changes were investigated over 24 hours and for up to 4 cell passages. Moreover, we were able to assay abdominal- thoracic skin samples from a laboratory worker who accidentally exposed himself to a high dose of SM, which we received one year after the initial exposure subsequent to a corrective surgical procedure. The results of our in vitro study clearly show changing epigenetic patterns over time, which partly coincide with the findings from the human skin samples exposed short-term to SM. Histone modifications generally remained fewer and DNA methylation increased significantly. Future investigations should focus on the confirmation of these results under in vivo conditions and include possible therapeutic interventions, e.g. DNA methyltransferase inhibitors, to prevent or reverse these effects.