In order to evaluate whether the lactic acidosis is developed during incremental exercise test in COPD. 12 untrained subjects with COPD performed incremental maximal cycle ergometer tests. Ventilation (VE); O2 uptake (VO2), CO2 output (VCO2); and end-tidal CO2 fraction (FETCO2) were measured. Arterial lactate concentration and blood gas analysis were measured at rest, maximal exercise and recovery of exercise from an indwelling arterial catheter. of the degree of airway obstruction. (2) T40 and BEecf correlated well with lactate change (r = 0.83; 0.84, P < 0.05). (3) The change of VE, and RQ correlated with the lactate change (r = 0.81; 0.83, 0.72, P < 0.05), but not the VE/VO2, VE/VCO2, nor FETCO2, (r = 0.24; 0.06; 0.46, P > 0.05). We concluded that the blood lactate increased markedly in patients of COPD during increment exercise, and it was well corrected with the change of VE, VCO2 and RQ. The increased lactate could be indicated by the concomitant change of T40 and BEecf.