The purpose of this study was to evaluate the reproducibility of visual analog scale ratings of the effort to breathe (VASe) and the degree of discomfort evoked by breathing (VASd) in patients with chronic obstructive pulmonary disease (COPD) during exercise. Six subjects with moderately severe COPD (FEV1 = 1.12 +/- 0.29 L, FEV1/FVC = 44 +/- 4%) underwent progressive incremental exercise testing to a symptom-limited maximum every week for 8 wk. VASe and VASd were highly correlated in each subject (r = 0.99 +/- 0.01). The slope of the VASd/VASe relationship for all trials in all subjects was not significantly different from 1, indicating that our subjects were rating a common sensation with the two scales. VASe at maximal exercise was reproducible in every subject; the within-subject coefficient of variation (CV) was 6% (range, 2 to 10%) and compared favorably with physiologic indices: 7% (range, 3 to 12%) for oxygen consumption and 10% (range, 5 to 16%) for minute ventilation (VI). In contrast, submaximal VAS ratings were highly variable. At 66% of the maximal work load, the within-subject CV for VASe was 21% (range, 11 to 28%) compared with 6% (range, 4 to 7%) for VO2 (p less than 0.003) and 10% (range, 5 to 16%) for VI (p less than 0.01). VASe correlated linearly with VI and VO2 in all subjects in all trials. However, within an individual subject the slope and position of these relationships varied widely between trials.(ABSTRACT TRUNCATED AT 250 WORDS)