Walking tests, such as the "shuttle" incremental walking test (SWT) and the 6-min walking test (6'WT), are commonly utilized in evaluating exercise intolerance in patients with chronic obstructive pulmonary disease (COPD) and the distance covered is the variable usually considered. Because lung gas exchange indexes are not measured, little is known about the physiological response elicited by different walking protocols. We compared exercise adaptation during the 6'WT and SWT in 13 male stable COPD patients [mean (SE) age: 70 (1) years; forced expiratory volume in 1 s (FEV(1)): 1.2 (0.1) l; arterial O(2) tension (PaO(2)): 72 (2) mmHg; arterial CO(2) tension (PaCO(2)): 41 (1) mmHg]. Oxygen uptake (.VO(2)), CO(2) output (.VCO(2)), minute ventilation (.V(E)), and heart rate (HR) were monitored by a portable telemetric system. During the SWT a linear response in lung gas exchange indexes was observed while, during the 6'WT, the response was exponential. During the 6'WT, .VO(2), .VCO(2), .V(E), and HR values at steady-state (SS) were significantly lower compared to SWT peak values. For SWT, distance covered correlated with .VO(2PEAK), (R=0.86, p<0.001), .VCO(2PEAK), (R=0.87, p<0.001) and .V(EPEAK) (R=0.74, p<0.01); moreover, distance and .VO(2PEAK) were significantly correlated with peak .VO(2) values obtained during cycle ergometer incremental exercise (R=0.72, p<0.01 and R=0.92, p<0.0001, respectively). For 6'WT, the distance covered did not correlate with any pertinent physiological index. The two walking protocols reveal substantial differences in pathophysiologic adaptations and provide evidence that SWT is more accurate than the 6'WT in the evaluation of maximal exercise tolerance in COPD patients.