BackgroundFamilial Mediterranean fever (FMF) is an autoinflammatory disease characterized by recurrent episodes of fever and serositis. Sacroiliitis can be observed in some FMF patients. This study aimed to compare the demographic, clinical, and laboratory findings, and treatment in children with FMF and sacroiliitis, and children with juvenile spondyloarthropathy (JSpA).MethodsIn total, 1687 pediatric FMF patients that were followed-up between May 2010 and June 2020 were evaluated retrospectively. Among them, those with sacroiliitis (n=63) were included in the study and compared to patients with JSpA (n=102).ResultsThe study included 63 FMF patients with sacroiliitis (38 males [60.3%] and 25 females [39.7%]) with a mean age of 15.24.1years. Mean age at symptom onset was 7.2 +/- 5.05years and mean age at diagnosis was 9.74 +/- 4.67years. The most common mutation in the FMF patients was M694V/M694V (n=22). Patients were diagnosed with sacroiliitis with a mean of 12months (range: 6-36months) after the diagnosis of FMF. Among the FMF patients, 28 (44.4%) had enthesitis, 23 (36.5%) had heel pain, and 11 (17.4%) had low back pain. The study also included 102 JSpA patients (90 males [88.2%] and 12 females [11.8%]). Mean age of patients with JSpA was 16.1 +/- 2.8years. As compared to 102 JSpA patients, patients with FMF and sacroiliitis had higher acute phase reactants, whereas HLA-B27 positivity rate was lower. In addition, axial involvement rate was higher in the JSpA patients.Conclusion Sacroiliitis is a common co-morbidity in FMF patients. The phenotypic features of these patients are different from patients with JSpA.