74 patients, 42 females, 32 men-all over 10 years of age, with acute arthritis associated with infection with Yersinia enterocolitica are presented. The diagnosis was based on serological evidence in all cases (an agglutinin titre of 320 or more), and in 18 of the cases also a positive culture of Y. ent. from faeces. The erythrocyte sedimentation rate was very high in most cases. Acute enteritis, sometimes with abdominal pain, preceded the acute arthritis with a latent period of about 1-14 days. In 50% of the cases only one or two joints were involved, most often large joints such as knee, ankle and wrist. The symptoms were long-standing (2 months or longer) in two-thirds of the cases. Arthritis in connection with yersiniosis must therefore be considered a serious complication to an otherwise rather benign infection.