BACKGROUND: Assessment of chest expansion is one of the measures of rib cage mobility recommended as "core set for assessing Ankylosing Spondylitis" (AS). A recently developed instrument for measuring respiratory movements is introduced. PURPOSE: To compare chest and abdominal wall movements in AS patients with those of controls using a newly developed instrument. STUDY DESIGN: A comparative study. PATIENT SAMPLE: Fourteen male AS patients were invited to the study. All subjects answered a standardized questionnaire concerning general health. OUTCOME MEASURES: Body height and weight and respiratory movements. METHODS: Upper and lower chest wall and abdominal motion was measured bilaterally for a period of 1 minute during deep breathing by using a new instrument based on a laser technique, The Respiratory Movement Measuring Instrument (ReMo, Reykjavík, Iceland). Results were compared to healthy controls matched for age, gender, and body mass index. RESULTS: The patients' mean age was 47+/-9.5 years, and they had a history of AS for 13+/-6 years. Their mean BMI was 27+/-3.6. The respiratory movements of the upper thoracic level were significantly lower than in the reference group (right p=.01, left p=.05). They had, however, a normal range of lower thoracic and abdominal movement and their respiratory movement patterns were symmetrical. CONCLUSION: The AS patients had reduced upper thoracic movements but normal lower thoracic and abdominal wall movements.