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Background: Enthesitis-related arthritis (ERA) is a subtype of juvenile idiopathic arthritis with high disease burden. The objectives of this study were to explore the prevalence of HLA-B27, clinical characteristics, and treatment outcomes in children with ERA and compare the differences between HLA-B27 positive and negative patients. Methods: A retrospective cohort study at a pediatric rheumatology clinic in a tertiary referral hospital in Bangkok, Thailand, including ERA patients with at least 6 months of follow-up (July 2011-April 2022) was performed. Data were collected from medical records from diagnosis to recent follow-up, assessing disease activity and treatment outcomes, with an analysis comparing HLA-B27 positive and negative patients. Descriptive statistics were used for data analysis. Results: There were 59 ERA patients with mean age ± SD at diagnosis 11.2 ± 2.5 years, 53 males (89.8%), and positive HLA-B27 in 38 patients (64.4%). The HLA-B27 positive group had significantly higher levels of inflammatory markers at initial diagnosis (p = 0.001), lower baseline hemoglobin (p = 0.001) and hematocrit (p = 0.002), higher disease activity assessed by the Juvenile Spondyloarthritis Disease Activity score at 6 and 12 months of follow-up (p = 0.028 and 0.040, respectively), increased utilization of bridging systemic corticosteroids (60.5% vs. 14.3%, p = 0.001) and anti-TNF (39.5% vs. 9.5%, p = 0.018), and longer duration of methotrexate (median[IQR] 1.7[1.1–3.1] vs. 1.3[0.6–1.9] years, p = 0.040). The HLA-B27 negative group had more prevalent hip arthritis than the positive group at initial diagnosis (66.7% vs. 28.9%, p = 0.005) and during the course of the disease (71.4% vs. 36.8%, p = 0.011). Conclusion: Most of the ERA patients tested positive for HLA-B27. Throughout the follow-up period, these patients demonstrated greater disease activity, greater use of corticosteroids and anti-TNF, and longer duration of methotrexate to control the disease. Key points: • HLA-B27 was tested positive in nearly two-thirds of enthesitis-related arthritis (ERA) patients. • ERA patients with positive HLA-B27 had significantly higher levels of inflammatory markers with lower hemoglobin and hematocrit at initial diagnosis, increased disease activity assessed by the Juvenile Spondyloarthritis Disease Activity (JSpADA) score at 6 and 12 months of follow-up, greater utilization of bridging systemic corticosteroids and anti-TNF, and longer duration of methotrexate use. • ERA patients with negative HLA-B27 had more prevalent hip arthritis at initial diagnosis and during disease course. • Application of the Assessment of SpondyloArthritis international Society (ASAS) classification for adult spondyloarthritis in ERA patients who entered adulthood showed the predominance of peripheral spondyloarthritis in more than half of the patients. [ABSTRACT FROM AUTHOR]