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Background: Reactive arthritis (ReA) is an inflammatory arthritis triggered by certain gastrointestinal and genitourinary infections. Single source outbreaks of triggering infections provide an opportunity to elucidate host susceptibility factors in this disease., Aim: To determine the role of Major Histocompatibility Complex (MHC) Class I alleles in ReA susceptibility after two large single source outbreaks of Salmonella Typhimurium gastroenteritis., Methods: A questionnaire screening for features of ReA and a request for HLA class I typing were sent to all patients affected by two single source outbreaks of S. Typhimurium gastroenteritis. Individuals with arthritis of recent onset were interviewed, examined and diagnostic criteria for ReA applied., Results: Nineteen cases of reactive arthritis, 11 female, were diagnosed in the 424 respondents with S. Typhimurium gastroenteritis from both outbreaks. Clinical features of the arthritis were similar to those described after other large single source outbreaks of Salmonella infection. HLA-B27 was expressed by only two of the 19 ReA patients and therefore did not predict susceptibility to this form of arthritis. Caucasians were, however, more likely to develop reactive arthritis than Asians., Conclusions: In this study, susceptibility to ReA was not increased in HLA-B27 positive individuals or males but was greater in those of Caucasian descent.