Part I reports pulmonary function and clinical measurements of 222 patients with ankylosing spondylitis, before and after physical therapy, to quantify thoracic impairment and to evaluate therapeutic effect. Vital capacity and forced expiratory volume remained virtually unchanged; cervical extensibility, lumbar flexibility, and chest expansion improved significantly. Part II reports findings in 25 patients and 25 age-matched normal subjects. The linear momentum of breathing was measured, from which the respiratory muscle balance (rib-cage vs. abdominal breathing) was derived. During normal breathing in supine position the mean diaphragmatic contribution to a tidal volume was 84.8 ± 3.3% SE in the patients and 68.4 ± 1.5% in the normal subjects. In 3 patients with severe ankylosis of cervical and thoracic vertebrae there was paradoxical movement of the chest, resulting in a diaphragmatic contribution of > 100%. An understanding of respiratory muscle function is essential to the formulation of rational physical therapy for patients with such conditions.