In July 2004, two individuals developed blisters after the destruction of a WWI-era munition. To determine the causative agent, urine samples were collected from both the highly blistered patient (patient 1; 6.5% of total body surface area) and patient 2, who had only one small blister. Their urine was analyzed for metabolites of known vesicants including sulfur mustard (HD), Lewisite (L1), and nitrogen mustards. The urine samples only tested positive for metabolites of HD. Additional metabolites were measured to confirm the exposure of sulfur mustard agent HD, including thiodiglycol (TDG), TDG-sulfoxide, and the bis-mercapturate of mustard sulfone. On day 2 after the exposure, patient 1 had a beta-lyase metabolite level of 41 ng/mL, and patient 2 had a level of 2.6 ng/mL. Detectable levels of the beta-lyase metabolite were observed in patient 1 for 11 days and in patient 2 for 7 days. Levels of TDG and both TDG and its sulfoxide measured together in the urine of patient 1 were found to be 24 ng/mL and 50 ng/mL, respectively, on day 2. The bis-mercapturate of mustard sulfone was detected in patient 1 (3.1 ng/mL) on day 2 but was not detected in samples taken on subsequent days.