Objectives Systemic-onset juvenile idiopathic arthritis (SJIA) and adult-onset Still’s disease (AOSD) are the same sporadic systemic auto-inflammatory disease. Spondyloarthritis (SpA) is a group of inflammatory non-autoimmune disorders. We report the observations of eight patients with SJIA/AOSD who also presented features of SpA during their disease evolution and estimate the prevalence of SpA in SJIA/AOSD. Methods This was a retrospective national survey of the departments of paediatric and adult rheumatology and internal medicine. To be included, SJIA patients had to fulfil the ILAR criteria, AOSD patients the Yamaguchi or Fautrel criteria, and all patients the ASAS classification criteria for axial or peripheral SpA, ESSG criteria for spondyloarthropathy or CASPAR criteria for psoriatic arthritis. The data were collected with a standardized form. Results Eight patients (five adults) were identified in one paediatric and two adult departments. In all but one patient, SpA manifestations occurred several years after SJIA/AOSD onset (mean delay 6.2 ± 3.8 years). Two patients had peripheral and three axial SpA, and four later exhibited psoriatic arthritis and one SAPHO syndrome. The prevalence of SpA in an adult cohort of 76 patients with AOSD was 6.58% (95% CI [2.17–14.69]), greater than the prevalence of SpA in the French general population (0.3%, 95%CI [0.17–0.46]). The prevalence of SpA in an SJIA cohort of 30 patients was 10% (95%CI [2.11–26.53]), more than that reported in the general population of industrialized countries, estimated at 0.016% to 0.15%. Conclusion Whilst the temporal disassociation between SpA and AOSD in most cases might suggest a coincidental finding, our work raises the possibility of an SpA AOSD spectrum overlap that needs further study.

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