Twelve children with spinal muscular atrophy (SMA) type 2 and 13 children without physical disability underwent kinematic analysis of thoracoabdominal volume changes when breathing spontaneously and when breathing deeply. A very accurate optical method of kinematic analysis was used. Volumes were partitioned into upper thoracic, lower thoracic, and abdominal compartments. Abdominal volume increases accounted for 96% of the normal tidal volumes and 87% of the deep breathing volumes for the patients, but only 74 and 41% of the volumes, respectively, for the controls. For the patients the upper thoracic contribution to breathing volumes was --1.7% for normal tidal volumes and 0.3% for deep breathing volumes. Patients with less upper thoracic kinematic reserve were also found to be more likely to have chronic nocturnal hypoventilation. We conclude that kinematic analysis can be helpful in determining differences in regional lung mobility and risk for nocturnal ventilatory dysfunction for children with SMA. Therapeutic interventions need to be addressed to maintain thoracic kinematic reserve and lung compliance and, thereby, to facilitate more normal lung growth and the ability to cough.