BACKGROUND--Several recent studies have reported associations between short term changes in air pollution and respiratory hospital admissions. This relationship was examined in two cities with substantially different levels of sulphur dioxide (SO2) but similar levels of airborne particles in an attempt to separate the effects of the two pollutants. Significant differences in weather between the two cities allowed the evaluation of that potential confounder also. METHODS--Daily counts of admissions to all hospitals for respiratory disease (ICD 9 460-519) were constructed for persons aged 65 years and older in two cities - New Haven, Connecticut and Tacoma, Washington. Each city was analysed separately. Average daily concentrations of SO2, inhalable particles (PM10), and ozone were computed from all monitors in each city, and daily average temperature and humidity were obtained from the US weather service. Daily respiratory admission counts were regressed on temperature, humidity, day of the week indicators, and air pollution. A 19 day weighted moving regression filter was used to remove all seasonal and subseasonal patterns from the data. Possible U-shaped dependence of admissions on temperature was dealt with using indicator variables for eight categories each of temperature and humidity. Each pollutant was first examined individually and then multiple pollutant models were fitted. RESULTS--All three pollutants were associated with respiratory hospital admissions of the elderly. The PM10 associations were little changed by control for either ozone or SO2. The ozone association was likewise independent of the other pollutants. The SO2 association was substantially attenuated by control for ozone in both cities, and by control for PM10 in Tacoma. The magnitude of the effect was small (relative risk 1.06 in New Haven and 1.10 in Tacoma for a 50 micrograms/m3 increase in PM10, for example) but, given the ubiquitous exposure, this has some public health significance. CONCLUSIONS--Air pollution concentrations within current guidelines were associated with increased respiratory hospital admissions of the elderly. The strongest evidence for an independent association was for PM10, followed by ozone. These results are consistent with other studies and suggest that lowering air pollution concentrations would have some impact on public health.