A case of an acute asymmetrical polyarthritis occurring in a teenage boy is described. This was shown by serological tests to be secondary to a recent infection with Yersinia enterocolitica. Reactive arthritis following infection by this organism is well recognised in Scandinavia. Only recently have two cases been reported in the U.K. (1,2). This is the first reported case in Scotland and is unusual in that the initial infection was asymptomatic. Clinical improvement was associated with falling Y. enterocolitica titres and a reduction in the E.S.R. The patient was HLA B27 positive. It is suggested that all patients presenting with an acute asymmetrical polyarthritis predominantly affecting the lower limbs should be screened by stool culture and serology for recent Y. enterocolitica infection.