We conducted a cross-sectional study in December 1994 in three metropolitan areas of the Rhone-Alpes region in France (Lyon, Grenoble, and Chambéry; total number of inhabitants = 970,000) to assess the medical costs resulting from exposure to particulate air pollution. Probability samples of the general population (508 families, 1265 subjects) and of the physicians (395) and 13 hospital respiratory care and emergency units in the study area provided data on the prevalence of respiratory disorders and on medical care usage. Measurements from air-quality monitoring networks were used to ascribe a fraction of the respiratory morbidity to the ambient air particle concentrations present during the study period, on the basis of attributable risk estimates drawn from recent meta-analyses. The medical care usage and absenteeism related to respiratory disorders were converted into direct and indirect medical and social costs by use of a "cost of illness" approach. These costs were extrapolated to annual costs of disease attributable to particulate pollution in 1994, using daily values of air pollution. The average particulate concentrations during the study period were moderate (39, 41, and 10 µg/m3 in the three cities), yielding attributable fractions that ranged between 0.6% and 13.8% according to the health condition and to the city. Three hundred ninety-five subjects reported respiratory symptoms (prevalence, 31.2%) during the study period; 1182 patients visited a doctor and 158 used hospital services. The extrapolated annual estimates of the attributable cost of respiratory diseases for a population of 1 million range between 79 and 135 million French francs (FF) (20th and 80th percentiles of the cost distribution, after a Monte Carlo simulation, respectively; 50th percentile, 107 × 106 FF [16.3 × 106 Euros]). Over-the counter drug consumption represents the largest cost item (approximately 44% of total costs), followed by wage losses (38%). Hospital expenditures amount to a low percentage of total costs (about 5%) because most respiratory disorders do not require hospital care. Mortality was not considered in this study. Most of these costs occur at relatively low levels of air pollution (67% of the total annual costs are incurred during days with particle concentrations lower than 50 µg/m3). Such substantial figures are useful for assessing the social impacts of air pollution and for evaluating the cost efficiency of abatement policies.