Few studies based in well-defined North American populations have examined the occurrence of juvenile idiopathic arthritis (JIA), and none has been based in an ethnically diverse population. We used computerized healthcare information from the Kaiser Permanente Northern California membership to validate JIA diagnoses and estimate the incidence and prevalence of the disease in this well-characterized population. We identified children aged ? 15 years with ? 1 relevant International Classification of Diseases, 9th edition, diagnosis code of 696.0, 714, or 720 in computerized clinical encounter data during 1996-2009. In a random sample, we then reviewed the medical records to confirm the diagnosis and diagnosis date and to identify the best-performing case-finding algorithms. Finally, we used the case-finding algorithms to estimate the incidence rate and point prevalence of JIA. A diagnosis of JIA was confirmed in 69% of individuals with at least 1 relevant code. Forty-five percent were newly diagnosed during the study period. The age- and sex-standardized incidence rate of JIA per 100,000 person-years was 11.9 (95% CI 10.9-12.9). It was 16.4 (95% CI 14.6-18.1) in girls and 7.7 (95% CI 6.5-8.9) in boys. The peak incidence rate occurred in children aged 11-15 years. The prevalence of JIA per 100,000 persons was 44.7 (95% CI 39.1-50.2) on December 31, 2009. The incidence rate of JIA observed in the Kaiser Permanente population, 1996-2009, was similar to that reported in Rochester, Minnesota, USA, but 2 to 3 times higher than Canadian estimates.