Sulfur mustard (2,2′-dichlorodiethyl sulfide), commonly known as mustard gas, was the “King of the War Gases.”2(p120)It was first used as a tactical weapon and a weapon of terror in World War I. Although there were many deaths, the numbers of nonfatal casualties were far greater, and the suffering of soldiers was legendary. In the first 3 weeks of its use, there were 14 000 nonfatal British casualties and 500 deaths, and large numbers were incapacitated. A vesicant, persistent gas, named for its odor, sulfur mustard’s effects were not immediate, becoming apparent 4 to 12 hours after exposure. Among its effects were skin blisters the size of one’s hand, blindness, and intractable vomiting and choking (often emerging after falling asleep on the night following exposure). With high concentrations, death generally occurred on the second or third day but could be drawn out over several weeks. John Singer Sargent’s Gassed, which hangs in the Hall of Remembrance at the British War Museum, is one of the most memorable and unsettling images of a mustard gas attack.