We evaluated isocapnic hyperventilation with room temperature gas (IHV) as a test of bronchial hyperreactivity and compared it with histamine challenge (HC) in three groups of subjects: normal subjects, known mild asthmatic patients, and patients referred to the pulmonary clinic for a chief complaint of dyspnea. Physical examination at the time of evaluation was negative in all subjects. When the criterion for a positive reaction to IHV was a decrease in FEV1 of 10 percent or greater, and the criterion for a positive reaction to HC was a PD20 (concentration of inhaled histamine necessary to decrease FEV1 by at least 20 percent) of less than 10 mg/ml, the specificity of both tests was 100 percent. The sensitivity of both tests as evaluated from the known asthmatic patients was also 100 percent. Among the dyspneic patients, eight of 30 reacted to both IHV and HC, two of 30 reacted to HC alone, and eight of 30 reacted to IHV alone. It was concluded that IHV compares favorably with HC as a test of bronchial hyperreactivity, patients with clinical histories highly suggestive for asthma might not need to undergo bronchial challenge testing, and neither HC nor IHV when used alone is able to identify all of the hyperreactive patients.