OBJECTIVE: Juvenile idiopathic arthritis (JIA) is a heterogeneous condition with very few clinical and laboratory signs that can help predict the course and severity of the disease in the individual patient. The cell-surface antigen HLA-B27 is well known to be associated with spondyloarthropathies, reactive arthritis, and enthesitis. HLA-B27 plays an important role in the classification of JIA, since evidence of sacroiliitis most often evolves after years of arthritis in other joints. We investigated the associations of HLA-B27 and the clinical manifestations of JIA using a method as close to a population-based study as possible. METHODS: We studied an incidence-based cohort of 305 patients collected prospectively in 3 Nordic countries (Sweden, Norway, Denmark). Clinical and serological data of the first 3 years of the disease were collected. RESULTS: HLA-B27 was found to be positive in 25.5% of the patients, and we found a higher proportion of HLA-B27-positive boys with older age at disease onset (p=0.034). Regression analysis showed a correlation of 0.7 in the HLA-B27-positive boys, pointing to a higher risk of more joint involvement with older age at disease onset. By Fisher's exact test, involvement of small joints in the lower extremities was associated with HLA-B27 in boys (p=0.011), but not in girls (p=0.687). HLA-B27 was associated with inflammatory back pain in both sexes (p=0.041 in boys, p=0.042 in girls), but with enthesitis only in boys (p