Diagnostic value of maximal exercise tidal volume. Though breathing pattern is frequently analyzed during clinical exercise testing, there is little information regarding its usefulness in the differential diagnosis of impaired exercise tolerance. This study tested the hypothesis that differences in peak tidal volume during exercise between patients with different cardiorespiratory diseases are related largely to differences in severity of respiratory mechanical impairment (vital capacity), not to differences in disease state. Patients with chronic obstructive pulmonary disease, restrictive lung disease, bronchial asthma, and heart disease (mitral valve disease or left ventricular dysfunction) were studied. Subjects selected had one and only one of the above diagnoses. All subjects performed maximal (symptom-limited) incremental exercise on a cycle ergometer. Multiple linear regression of all subjects (n = 30) in all four groups showed a significant correlation between VTmax and VC: VTmax = 0.55, VC -0.09 L (r = 0.827, p less than 0.0001). The VTmax/VC (x 100) was (mean +/- SD) 44 +/- 15, 54 +/- 11, 56 +/- 11, and 54 +/- 12 for the COPD, RLD, BA and HD patients respectively. There was no significant difference between any of the groups. We concluded that differences in VTmax between different patients are related largely to differences in VC (ie, differences in severity of respiratory mechanical impairment), not to differences in disease state. Measurement of VTmax or the VTmax/VC ratio has little value in the differential diagnosis of exertional dyspnea. 