We evaluated the effect of breathing 100 ppm of carbon monoxide versus compressed, purified air for 1 hour on exercise performance in 10 patients with chronic obstructive pulmonary disease in a double-blind, randomized, crossover study. The mean arterial carboxyhemoglobin was 1.48 per cent in the carbon monoxide control period and increased from 1.43 to 4.08 per cent after breathing carbon monoxide (P less than 0.001). The mean arterial carboxyhemoglobin level was 1.52 percent in the air control period and decreased from 1.47 to 1.34 per cent after purified air (P less than 0.001). The mean exercise time until marked dyspnea decreased from 218.5 seconds in the carbon monoxide control period to 146.6 seconds after breathing carbon monoxide (P less than 0.001). The mean exercise time was 219.9 seconds in the air control period and 221.3 seconds after purified air (P not significant). Breathing 100 ppm of carbon monoxide for 1 hour caused a significant reduction in exercise performance in patients with chronic obstructive pulmonary disease.