Several studies in recent years have suggested that exposure to airborne particles and to ozone are associated with increases in respiratory hospital admissions. Few of those studies have used inhalable particles as their measure of exposure, and the studies did not always examine both particle and ozone exposure. This study examined the association between both PM10 and ozone and respiratory hospital admissions for persons 65 yr of age and older in the Detroit, Michigan, metropolitan area during the years 1986 to 1989. After controlling for seasonal and other long-term temporal trends, temperature, and dew point temperature, both PM10 (RR = 1.012, 95% CI = 1.019-1.004) and 24-h ozone concentrations (RR = 1.026, 95% CI = 1.040-1.013) were associated with daily admissions for pneumonia. The relative risks are for a 10-microgram/m3 increase in PM10 and a 5-ppb increase in 24-h ozone concentration and from models including both pollutants. Admissions for COPD other than asthma were associated with PM10 (RR = 1.020, 95% CI = 1.032-1.009) and ozone (RR = 1.028, 95% CI = 1.049-1.007) as well. Asthma admissions were not associated with either pollutant. Controlling for one pollutant did not effect the magnitude of the association with the other pollutant. The magnitude of these relative risks are very similar to those recently reported in Birmingham, Alabama, Ontario, and New York State. This suggests that the associations with both pollutants are likely to be causal.