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OBJECTIVE: To determine the prevalence of ossification of the posterior longitudinal ligament (OPLL) in patients with ankylosing spondylitis (AS). METHODS: A cross-sectional radiological examination was performed in patients diagnosed with AS. A bone and joint radiologist screened and confirmed the cervical radiographs of these patients. A review of the medical records was also conducted to investigate the relationship between the clinical manifestations and the actual expression of OPLL in patients with AS. RESULTS: Among 544 patients with AS, 470 (86.4%) were men and 96.1% were HLA-B27-positive. The mean age was 34.3 +/- 9.3 years and mean disease duration was 12.4 +/- 7.2 years. After reviewing the cervical radiographs, OPLL was found in 19 patients (3.5%; 95% CI 1.9, 5.0). The mean age of these 19 patients was 39.9 +/- 10.7 years, and the male to female ratio was 18:1. Interestingly, a statistically significant number of patients who expressed OPLL were older (p = 0.007). However, we were unable to determine whether OPLL was significantly associated with AS disease duration, peripheral arthritis, anterior uveitis/iritis, HLA-B27, anterior atlantoaxial subluxation, diffuse idiopathic skeletal hyperostosis, or other paraspinal ligament disorders. CONCLUSION: Our study showed that the frequency of OPLL in Korean patients with AS was 3.5%, which was considerably lower than previously reported values (15.5% in 103 Mexican AS). We were able to determine that OPLL in AS was associated with older age.