An outbreak of infection with Salmonella typhi murium occurred in southern Sweden in the summer of 1974. About 330 persons were infected; 91 were hospitalized. 13 of the 91 patients (9 men and 4 women) had symptoms of aseptical polyarthritis. The 13 patients with arthritis were HLA typed with a standard microlymphocytotoxicity test. 13 salmonellosis patients from the same epidemic but without arthritis and 446 healthy blood donors served as controls. Nine (69%) of the 13 patients with arthritis had HLA-B27 compared to 1 (8%) of the control patients and 44 (10%) of the blood donors. The observed association between HLA antigen B27 and reactive arthropathy following salmonella infection could be due to a function of a disease predisposing gene closely linked to HLA-B27.