The total annual incidence of arthritis in this prospective cross-sectional study on adults was 115/100 000. The annual incidence of rheumatoid arthritis (RA) was 24/100 000, 29/100 000 for women, and 18/100 000 for men. For reactive arthritis (ReA) the annual incidence was slightly higher, 28/100 000, and for undifferentiated arthritis 41/100 000. The annual incidence of Lyme disease and sarcoid arthritis was low. The annual incidence of arthritis in this study compares well with findings in earlier reports from both registers and case review studies. Almost 50% of the patients in the series of 71 patients with arthritis of less than 3 months’ duration had a preceding infection. Campylobacter jejuni ReA dominated the enteric ReA group. We found only a few patients with preceding Chl. trachomatis, Chl. pneumoniae, Borrelia burgdorferi or parvovirus B19 infections. The arthritis patients with a preceding infection went into remission more often than the patients without a preceding infection. The disease specificity of anti-CCP antibodies for RA was high, 96%, confirming earlier results. Anti-CCP antibodies differentiated RA from other arthritides. Several patients in the different diagnosis groups had raised serum COMP levels, indicating cartilage involvement very early in the disease, even in mild and self-limiting disease with good prognosis. The economic burden of early joint inflammation was found to be considerable already during the first few months of the arthritis irrespective of diagnosis. Surprisingly, patients with ReA generated almost as high costs as patients with RA during thefirst few months of the disease, even though most of the ReA patients had a relatively mild disease. Sick leave accounted for about 50% of the costs. The distribution of costs in the different patient groups was skewed. The median cost per patient for the group of patients with RA was US$4385, for ReA US$4085, for other types of specified arthritis US$3361, and for undifferentiated arthritis US$1482. This underlines the necessity of quick referral and therapy, not only to decrease the inflammation and prevent functional impairment, but also to decrease the costs of early arthritis.