OBJECTIVE To determine the prevalence of anterior and vertical atlantoaxial subluxation (AAS) and its neurological effect in a consecutive sample of Mexican patients with ankylosing spondylitis (AS). METHODS Consecutive patients with a diagnosis of AS who attended 2 secondary care outpatient rheumatology clinic in the city of Guadalajara within a period of 6 mo were included in the study. Patients had prospective rheumatologic and neurologic assessments using structured questionnaires and examinations. Recorded variables included demographic data, disease characteristics, neurologic symptoms and signs, and axial mobility measurements. Somatosensory evoked potentials (SSEP) were performed in all patients. Patients also underwent standard cervical spine radiography with anteroposterior open mouth, and neutral, full flexion and extension lateral views. RESULTS We studied 103 patients. Mean age was 35 yrs, 74% were male, and mean disease duration was 10 yrs (S/- SD 7.9). Anterior AAS was observed in 22 patients (21%, 95% CI 13-29%) and vertical AAS in 2 cases (2%, 95% CI 0-7%). Ossification of the posterior longitudinal ligament (OPLL) was present in 16 patients. Statistically significant associations were observed between anterior AAS and SSEP (p < 0.0001) and OPLL (p = 0.04). The degree of radiologic sacroiliitis was also associated with anterior AAS. After completion of the study, 2 patients required surgical fusion due to severe cervical cord compression. CONCLUSIONS The prevalence of AAS and OPLL in this population was higher than previously reported in other settings. The association of anterior AAS with OPLL and radiological sacroiliitis could identify a subgroup of patients with more severe axial enthesopathy. Although clinically significant neurologic complications are not frequent in these patients, AAS may cause severe spinal cord compression requiring surgical fusion.