Few studies have been conducted on the effects of air pollution on patients with chronic obstructive pulmonary disease (COPD). During a 14-mo period, 39 Parisian adults with severe COPD were monitored by their physicians. Daily levels of 4 air pollutants were provided by an urban air-quality network. Exacerbation of COPD was associated only with ozone (O3) (odds ratio [OR] = 1.44 for a 10-microg/m3 increase in O3; 95% confidence interval [CI] = 1.14, 1.82), with a lag of 2-3 days. The effect of O3 was greater in patients whose carbon dioxide pressure (PaCO2) was higher than 43 mm Hg (OR = 1.83; 95% CI = 1.36, 2.47) vs. those with a lower PaCO2 (OR = 1.26; 95% CI = 0.90, 1.77). The effect of O3 was unchanged, regardless of the maintenance medications used. The only air pollutant to which patients with severe COPD were particularly sensitive was O3.