The main purpose of this study was to examine the relative contribution of respiratory mechanical factors and the increased metabolic cost of locomotion to exertional breathlessness in obese women. We examined the relationship of intensity of breathlessness to ventilation (V'_E) when exertional oxygen uptake (V'O₂) of obesity was minimized by cycle exercise. Eighteen middle-aged (54±8 years, mean±SD) obese (body mass index [BMI] 40.2±7.8 kg/m²) and 13 age-matched normal-weight (BMI 23.3±1.7 kg/m²) females were studied. Breathlessness at higher submaximal cycle work-rates was significantly increased (by 1 Borg unit) in obese compared to normal-weight females, in association with a 35-45% increase in V'_E and a higher metabolic cost of exercise. Obese females demonstrated greater resting expiratory flow limitation, reduced resting end-expiratory lung volume (EELV)(by 20%) and progressive increases in dynamic EELV during exercise: peak inspiratory capacity (IC) decreased by 16% (0.39 L) of the resting value. V'_E/V'O₂ slopes were unchanged in obesity. Breathlessness ratings at any given V'_E or V'O₂ were not increased in obesity, suggesting that respiratory mechanical factors were not contributory. Our results indicate that in obese females, recruitment of resting IC and dynamic increases in EELV with exercise served to optimize operating lung volumes and to attenuate expiratory flow limitation so as to accommodate the increased ventilatory demand without increased breathlessness.