Sulphur mustard (SM) is regarded as one of the most important agents of chemical warfare because of its simple and cheap chemical synthesis that makes it readily available for both terrorist and military use. SM acts as an alkylating agent that induces disruption of nucleic acids and proteins, impairing cell homeostasis and eventually causing cell death. It rapidly reacts with ocular, respiratory and cutaneous tissues, as well as bone marrow and the mucosal cells of the gastrointestinal tract, resulting in several devastating long-term effects on human health, many of which are not clinically or pathologically well defined. In light of the possible threat of SM use against military and civilian populations, physicians should be aware of its grave effects and knowledgeable how to care for its victims. The pattern of immediate and long-term toxic effects following exposure to SM is reviewed in this article with special references to the recent data available from over 100,000 chemical casualties incurred during the Iran-Iraq conflict.