Background. There are no studies evaluating the possible association between ankylosing spondylitis (AS) and apical periodontitis (AP). Objectives. The aim of the present cross-sectional study was to investigate the possible association between AS and AP. Material and methods. Fifty patients diagnosed with AS, receiving treatment at the Rheumatology Clinic in Erzurum, Turkey, were included in the experimental group. Another 50 age-and gender-matched individuals without any history of systemic disease were included in the study as the control group. All patients were examined radiographically and clinically to diagnose the presence of AP. The following data was recorded for all patients: the smoking habit, the number of teeth present, the number of teeth with AP, the number of root canal-treated (RCT) teeth, and the number of RCT teeth with AP. Results. There were 1,283 teeth in the AS group and 1,305 in the control group. There was a significant association between teeth with AP and AS, as the prevalence of teeth with AP was significantly lower in the control group (1.3%) than in the AS group (2.9%) (OR (odds ratio) = 2.250; p = 0.005). There was no statistically significant difference between the groups in terms of the number of RCT teeth and RCT teeth with AP (p > 0.05). Conclusions. Ankylosing spondylitis is significantly associated with an increased prevalence of AP. It can be concluded that patients with AS can be more prone to develop AP. However, AS does not reduce the success rate of endodontic treatment, because there was no significant difference between the AS and control groups in terms of RCT teeth with AP.