Aim The objective of this study was to perform a clinical and ultrasono-graphic assessment of foot disability and related factors among patients with ankylosing spondylitis. Material and Method: The study enrolled 40 patients diagnosed with ankylosing spondylitis (AS) according to the modified New York criteria and 30 matched healthy controls. In addition to the assessments for Disease activity (BASDAI) and functional status (BASFI), foot functioning was evaluated using the Foot Function Index (FFI) and quality of life using the Ankylosing Spondylitis Quality of Life (ASQoL) questionnaire. Thickness of plantar fascia (PF) and Achilles tendon (AT), changes in echogenicity and presence of bone erosions, entesophytes and bursitis were examined using ultrasound. Results: The mean age of patients was 39.9 +/- 10.4 years and median disease duration was 48 (1-288) months. Sixteen patients (40%) had foot pain. Thirteen patients (32.5%) had clinical evidence for enthesitis. Thirty patients (75%) showed at least one pathological finding at ultrasonographic examination. Mean FFI score was higher in the AS group versus control group (p<0.001). Mean PF and AT thickness values were found to be greater in the patient group (p<0.05 and p<0.001, respectively). A significant positive correlation was found between FFI scores and BASDAI, ASQoL and BASFI scores among patients (all p values<0.001). Discussion: Foot involvement is a major cause of disability in AS patients. Foot disability is associated with active disease and results in reduced quality of life.