Bronchodilatory response to inhaled fenoterol was studied in 15 hyperthyroid patients before and after successful treatment with antithyroid drugs. Baseline forced vital capacity (FVC) and forced expiratory volume in 1 sec (FEV1) were lower than the predicted values in 12 and 11 patients, respectively. Improved values were seen after treatment for hyperthyroidism although statistical significance was not reached. Even if some improvement occurred in PEFR (a rise by 0.24-0.48 L/s) and FVC (increase of 73-78 ml) in the hyperthyroid state in response to fenoterol inhalation after various time intervals, the increase in different parameters of lung function was significantly more after the patients achieved euthyroid state (increases in FVC by 290-165 ml; in FEV1 by 333-193 ml; in peak expiratory flow (PEFR) by 0.75-0.52 L/s and in forced expiratory flow (FEF50%) by 0.55-0.31 L/s). In the euthyroid state the mean absolute improvements from the baseline values were significantly higher (< 0.05-0.001). These observations indicate that bronchodilatory response is impaired in the presence of excess thyroid hormones and improves after euthyroid state is achieved.