The purpose of the study was to characterise statistically the inherent fluctuations in breath-by-breath measurements of pulmonary gas exchange (oxygen uptake and carbon dioxide output, V*O2 and V*CO2, respectively) and pulmonary ventilation (V*E) in patients with chronic obstructive pulmonary disease (COPD) and to compare them with those of healthy control subjects. Thirty subjects with COPD [mean (SD): 67 (6) years old; forced expiratory volume in 1 min, FEV1 1.25( 0.18) l; 42 (6)% predicted FEV1] and 12 healthy subjects [31 (3) years old; FEV1 3.62 (0.54) l; 99 (8)% predicted FEV1] performed exercise tests on a cycle ergometer at a constant work rate of moderate intensity. Steady-state exercise values for V*O2, V*CO2 and V*E were 905 (96) ml.min(-1), 847(90) ml.min(-1) and 23 (3) l.min(-1), respectively for the COPD patients and 1239(89) ml.min(-1), 1191(84) ml.min(-1)and 37(3) l.min(-1), respectively, for the healthy controls. The breath-by-breath fluctuations were well characterised by a Gaussian density-probability function with breath-to-breath autocorrelations that were not significantly different from 0, up to four subsequent breaths. Its magnitude varied among variables, but was independent of the signal amplitude for the same subject and variable. With ratios of amplitude of fluctuation:signal of around 10%, typical of the patients studied, the resolution of time constants and amplitude were congruent with 9 s and congruent with 100 ml.min(-1), respectively for V*O2 or V*CO2 with one repetition.