Pulmonary function, arterial blood gases, and pulse rate were prospectively compared with dyspnea perceived by patients before and after emergency room treatment for acute asthma in 83 episodes. Subjective degree of dyspnea was rated on a modified Borg scale. Before treatment, all spirometry (FVC, FEV1, and peak expiratory flow rate) and arterial blood gas (PaO2, PaCO2, and bicarbonate) data as well as pulse rate were significantly correlated with modified Borg scale, but only PaCO2 showed significant correlation (P < .001) in a multivariate analysis. After treatment, only PaO2 was significantly correlated with modified Borg scale (P = .008) in a multivariate analysis, and pulse rate showed correlation of borderline significance (P = .06). In another 37 episodes of relapsed acute asthma, the correlations between laboratory data and modified Borg scale were very weak, especially after treatment. We conclude that dyspnea expressed by asthmatic patients in an emergency room might be reflecting different mechanisms as treatment is performed. Although it should be one useful guide in emergency evaluation, it must be recognized that dyspnea in the same patient could mean different abnormalities.