Incidence and outcome of inflammatory joint disease (IJD) were studied by an epidemiologically based follow-up study of unselected adult arthritis patients. The annual incidence of all IJD together was 2.18% in the population of 16 years and older; 5.67% in the HLA B27 positive and 1.62% in the B27 negative population. Almost half of all arthritides were of non-specific nature. Incidence of definite rheumatoid arthritis was 0.42% per year. The outcome after 8 years was worst in seropositive rheumatoid arthritis and best in seronegative oligoarthritis of unknown etiology. Outcome of seronegative polyarthritis was bimodal: majority of patients with a favourable and minority with a poor outcome. In Yersinia arthritis, patients with a mainly IgA-class antibody response had a more severe clinical picture than those with a mainly IgG-class response. Diagnosis of rheumatoid arthritis at the early stage of the disease should be based on specific features like rheumatoid factor, radiological signs, symmetric peripheral polyarthritis and morning stiffness with a long duration. Erosive rheumatoid arthritis is almost always seropositive, at least at some stage of the disease.