The strength of the association between outdoor air pollution and hospital admissions in children has not yet been well defined. To estimate the impact of outdoor air pollution on respiratory morbidity in children after controlling for the confounding effects of weather, season, and other pollutants. The study used data on respiratory hospital admissions in children (three age groups: < 1, 1-4, and 5-14 years) for five cities in Australia and two in New Zealand. Time series of daily numbers of hospital admissions were analyzed using the case-crossover method; the results from cities were combined using a random-effects meta-analysis. Significant increases across the cities were observed for hospital admissions in children for pneumonia and acute bronchitis (0, 1-4 years), respiratory disease (0, 1-4, 5-14 years), and asthma (5-14 years). These increases were found for particulate matter with a diameter less than 2.5 microm (PM2.5) and less than 10 microm (PM10), nephelometry, NO2, and SO2. The largest association found was a 6.0% increase in asthma admissions (5-14 years) in relation to a 5.1-ppb increase in 24-hour NO2. This study found strong and consistent associations between outdoor air pollution and short-term increases in childhood hospital admissions. A number of different pollutants showed significant associations, and these were distinct from any temperature (warm or cool) effects.