Juvenile idiopathic arthritis (JIA) is the most common chronic pediatric rheumatic disease and the most common cause of acquired disability in childhood in the US [1–6]. Incidence and prevalence rates vary significantly by geographical location, with an estimated annual incidence of approximately one per 100 000 children in Japan , 90 per 100 000 children in the US , and 170 per 100 000 in Belgium . These widely disparate rates may reflect differences in genetic susceptibility, environmental influences, and other unknown factors. The term JIA encompasses seven categories of disease that involve arthritis of at least one joint that begins before the age of 16 years and lasts ≥6 weeks. Distinct clinical features characterize each of the JIA categories during the first 6 months of disease. The enthesitis-related arthritis (ERA) category of JIA describes a heterogeneous group of children, including those with enthesitis and arthritis, inflammatory bowel disease (IBD)-associated arthropathy, and what is traditionally thought of as juvenile ankylosing spondylitis (AS). ERA accounts for approximately 15–20% of JIA cases and has a peak age of onset of 12 years [10–13]. Boys are affected more often than girls, accounting for 60% of cases . Approximately half of children are human leukocyte antigen-B27 (HLA-B27)-positive and 20% have a family history of HLA-B27-associated disease, such as reactive arthritis, AS, or IBD with sacroiliitis. Tests for rheumatoid factor (RF) and anti-nuclear antibodies (ANA) are characteristically negative in patients with ERA. ERA is similar to but not interchangeable with juvenile spondyloarthropathy (SpA). Juvenile SpA includes not only children with ERA but also many who do not meet ERA criteria, including patients with the International League against Rheumatism (ILAR) undifferentiated and psoriatic arthritis (PsA) categories, IBD-related arthritis, juvenile AS, and reactive arthritis.