Yersinia arthritis is an acute oligo- or, more rarely, polyarthritis. It occurs after pyrexic diarrhoea of several days, often associated with right-sided lower abdominal pain. The disease is caused by Yersinia enterocolitica and four such cases, confirmed serologically in three, are described. In one case fluid from the knee-joint was antibody-positive. It was not possible to culture the causative organism, probably because all patients had previously been treated with an antibiotic. Rheumatic fever and Reiter's disease must be considered in the differential diagnosis. Because pharyngitis, transitory ECG changes and conjunctivitis may occur in the early stages of Yersinia arthritis, the diagnosis may be missed. Prognosis of Yersinia arthritis is good, with complete cure within weeks or months. Demonstration of HLA-B 27 in all four cases points to a genetic disposition.