In order to investigate the mechanism of dyspnoea in hyperthyroidism measurements of spirometry, lung volume, transfer factor for carbon monoxide and its subdivisions, maximal respiratory pressures, methacholine challenge, arterial blood gases were made and exercise studies performed on 16 patients before treatment for hyperthyroidism. Methacholine challenge showed that only three of 14 patients increased airway reactivity, which was mild. Maximal pressures which could be generated by the respiratory muscles were reduced in some patients, as was functional residual capacity. Exercise ventilation and breathing frequency were increased and the respiratory exchange ratio was abnormally high. Anaerobic threshold was measured in nine of 15 subjects and was below normal in each case. All but two subjects stopped exercise because of dyspnoea, and the maximum oxygen uptake achieved by the group was 53 per cent (n = 15, range 26-66 per cent) of predicted maximum oxygen consumption. The maximum ventilation averaged only 43 per cent (n = 15, range 16-96 per cent) of the maximal breathing capacity predicted from spirometric tests. Nine patients were studied shortly after being rendered euthyroid by treatment. At rest, only maximal respiratory pressures increased significantly. On exercise, the maximal workload attained and the ventilation achieved increased significantly. Breathing patterns, maximal oxygen consumption, ventilation, anaerobic threshold and cardiac frequency remained unchanged. We conclude that: patients with hyperthyroidism do not generally have increased airway reactivity; when hyperthyroid, respiratory muscles are weak, and improve following treatment; exercise capacity is impaired in hyperthyroid patients probably because of a combination of an inefficiently rapid and shallow breathing pattern, an increase of anaerobic metabolism and discomfort associated with the act of breathing. Although exercise capacity increases and the sensation of dyspnoea may decrease after treatment the pattern of breathing does not immediately return to normal.