Breathlessness, disability, and exercise tolerance were assessed in 26 patients with severe chronic airflow limitation (forced expiratory volume in one second (FEV1) ≤ 1 litre) divided into two groups - 15 patients who were normocapnic (pressure of arterial carbon dioxide (PaCO2) 6 kPa (>45.1 mm Hg)). The two groups were well matched for spirometric values (FEV1 0.59 l and 0.62 l, respectively). All of the hypercapnic patients could improve blood gas tensions towards normal by hyperventilation. There were no significant differences in visual analogue scores of breathlessness during treadmill exercise, disability (oxygen-cost diagram, dyspnoea grade), or exercise tolerance (six-minute walk, maximal consumption of oxygen during bicycle ergometry, distance walked to exhaustion in progressive treadmill test). The findings show that the 'fight' to maintain normal blood gas tensions in the face of severe airflow limitation does not have an appreciable cost in terms of disability.