1. The pattern of breathing in 12 patients with severe irreversible airflow obstruction has been studied during ventilatory stimulation by rebreathing CO2. Mean maximum tidal volume response was only 1·23 ± 0·30 litres (mean ± sd); this represented 65% of mean measured vital capacity and 82% of mean measured inspiratory capacity. During the course of rebreathing mean total breath duration was reduced from 3·48 ± 0·93 to 2·44 ± 0·48 s.
2. End-expiratory thoracic gas volume (FRC) was elevated at rest in all subjects and increased significantly by a further 0·50–1·90 litres during ventilatory stimulation in 10 of the 12 subjects. The maximum increase in FRC was proportional to the degree of airflow obstruction afforded by the airways in each subject.
3. It is suggested that the increase in FRC during ventilatory stimulation is responsible for the diminished tidal volume response and is an important determinant of breathing pattern and symptomatology in patients with airflow obstruction.