Involvement of the costovertebral (CV) and costotransverse (CT) as well as the sacroiliac (SI) joints is known to occur in patients with ankylosing spondylitis (AS). The functional significance of these changes is not clear. We have performed clinical and radiological evaluations and assessed the effect of joint involvement on pulmonary function. We detected radiologic evidence of involvement of the CV joint in 80% of patients and of the CT joint in 60 %. We found a direct relation between the severity of CV, CT and SI joint affliction, and the severity of CV and SI joints were related to time of evolution of the disease. Pulmonary function tests revealed neither restrictive nor obstructive defects. No relation was found between pulmonary function and CV and CT joint affliction. Patients with stiffer spines had a tendency to have pulmonary function tests within the lower limit of the normal range. In patients with AS diaphragmatic breathing might compensate the chest respiration to some extent.