Ventilatory gas exchange during exercise was compared in patients with mild asthma (11 females and 11 males), hyperventilation syndrome (HVS, 11 females), and healthy subjects (11 females and 11 males) in order to assess hyperventilation during exercise and its association with exercise-induced bronchoconstriction. The asthmatics showed decreased working capacity and decreased maximal oxygen consumption, with no evidence of limitation due to impairment of ventilatory capacity. Ventilatory equivalents for CO2 and O2 (VE/VCO2 and VE/VO2) at rest did not differ between the controls and asthmatics, but they were significantly elevated in HVS. In female asthmatics, ventilatory equivalents during exercise were significantly (P < 0.05) elevated compared with those of healthy subjects; in female controls, VE/VCO2 was 30.1 +/- 3.3 at low exercise and 27.4 +/- 6.5 at maximal exercise. In female asthmatics, the corresponding figures were 34.9 +/- 6.1 and 36.7 +/- 5.3. Furthermore, VE/VCO2 individually related to percent of maximal oxygen consumption (VO2max) was significantly increased in female asthmatics both at low and high VO2. The highest ventilatory equivalents were obtained in HVS, 41.7 +/- 6.7 and 43.9 +/- 0.9, respectively. Significant exercise-induced bronchoconstriction (decrease of FEV1 > 15%) was found in 50% of the asthmatics. The ventilatory equivalents did not correlate with exercise-induced changes in FEV1 (r2 < 0.3). Mild exercise-induced hyperventilation which was observed in mild female asthmatics, did not appear to be related to exercise-induced bronchoconstriction.