The objective of this study was to investigate the frequency of sacroiliitis in familial Mediterranean fever (FMF) patients and the role of HLA-B27 and MEFV mutations in the development of sacroiliitis. The study group consisted of 256 FMF patients (male 128, female 128, mean age 27.2 +/- 6.3 years). After evaluation of the medical records, 70 patients (27.4%) were determined to have one or more of musculoskeletal manifestations. Sacroiliitis was determined in 18 (32.7%) FMF patients. The frequency of sacroiliitis among all FMF patients was found to be 7%. HLA-B27 was 47% and 6.3% in FMF patients with and without sacroiliitis, respectively. The frequency of M694V mutations in FMF patients with sacroiliitis was 93.7%. Sacroiliitis may be seen more frequently in FMF patients than expected. HLA-B27 positivity and/or M694V mutation may play a role in the development of sacroiliitis and the severity of seronegative spondyloarthropathy.