Recent evidence has implicated the fine fraction of particulate as the major contributor to the increase in mortality and morbidity related to particulate ambient levels. We therefore evaluated the impact of daily variation of ambient PM2.5 and other pollutants on the number of daily respiratory-related emergency visits (REVs) to a large pediatric hospital of Santiago, Chile. The study was conducted from February 1995 to August 1996. Four monitoring stations from the network of Santiago provided air pollution data. The PM2.5 24-hr average ranged from 10 to 111 micrograms/m3 during September to April (warm months) and from 10 to 156 micrograms/m3 during May to August (cold months). Other contaminants (ozone (O3), nitrogen dioxide (NO2), and sulfur dioxide (SO2)) were, in general, low during the study period. The increase in REVs was significantly related to PM10 and PM2.5 ambient levels, with the relationship between PM2.5 levels and the number of REVs the stronger. During the cold months, an increase of 45 micrograms/m3 in the PM2.5 24-hr average was related to a 2.7% increase in the number of REVs (95% CI, 1.1-4.4%) with a two-day lag, and to an increase of 6.7% (95% CI, 1.7-12.0%) in the number of visits for pneumonia with a three-day lag. SO2 and NO2 were also related to REVs. We conclude that urban air pollutant mixture, particularly fine particulates, adversely affect the respiratory health of children residing in Santiago.