To describe the patterns and time course of arthritis in patients with antinuclear antibody (ANA)-positive juvenile idiopathic arthritis (JIA). We identified patients followed during a 16-year period who had JIA by ILAR criteria, were ANA-positive (i.e., had >or= 2 positive ANA test results at titer >or= 1:160), and had a disease duration >or= 2 years. Demographic and clinical features, including ILAR category and cumulative number and type of joints affected over time, were recorded. A total of 195 patients were studied. The ILAR category was oligoarthritis in 159 patients and rheumatoid factor-negative polyarthritis in 36 patients. The cumulative rate of polyarticular extension in patients with oligoarticular onset was 26%, 38%, 45%, 49%, and 51% at 1, 2, 3, 4, and 5 years, respectively. At disease onset, most patients had monoarthritis and 95% had <or= 4 joints affected. The knee was the most frequently involved of all joints, followed by the ankle and proximal interphalangeal joints. Among patients with oligoarticular onset, the presence of ankle (in case of monoarticular disease) and/or wrist involvement in the first 6 months was more common in those who progressed to polyarthritis. The majority of our ANA-positive patients, including most of those who later developed polyarthritis, had monoarthritis at disease onset. Among patients with oligoarticular onset, polyarticular extension occurred in around 50% of cases within the first 3-4 years after disease onset, and tended to be less likely thereafter. The early occurrence of ankle and/or wrist disease may indicate a higher likelihood of arthritis progression.