The acute clinical picture and long-term prognosis of 160 patients with Reiter's disease (RD), and 144 patients with reactive arthritis triggered by yersinia infection (YA), was analysed. Most of the patients with RD were men, while YA was as common among male and female patients. YA manifested in a third of patients as incomplete or complete RD. The long-term prognosis in RD was less favourable than in YA. Recurrent arthritis, development of chronic destructive arthritis and radiologic sacroiliitis were more frequent in RD than in YA. The presence of HLA-B27 was the major factor determining extra-articular inflammatory symptoms and the development of chronic problems.