Many epidemiological studies conducted in the last several years have reported associations between exposure to airborne particulate matter, measured as PM10 (<10 microm in diameter), and daily morbidity and mortality. However, much of the evidence involves effects on the elderly population; there is less evidence about the effects of particulates on children, especially those under 2 years of age. To examine these issues, we conducted time-series analyses of 2 years of daily visits to primary health care clinics in Santiago, Chile, where counts were computed for either upper or lower respiratory symptoms and for cohorts of children 3-15 years of age and below age 2. Daily PM10 and ozone measurements and meteorological variables were available from instruments located in downtown Santiago. The multiple regression analysis indicates a statistically significant association between PM10 and medical visits for lower respiratory symptoms in children ages 3-15 and in children under age 2. PM10 is also associated with medical visits related to upper respiratory symptoms in the older cohort, while ozone is associated with visits related to both lower and upper respiratory symptoms in the older cohort. For children under age 2, a 50- microg/m3 change in PM10 (the approximate interquartile range) is associated with a 4-12% increase in lower respiratory symptoms. For children 3-15 years of age, the increase in lower respiratory symptoms ranges from 3 to 9% for a 50- microg/m3 change in PM10 and 5% per 50 ppb change in ozone. These magnitudes are similar to results from studies of children undertaken in Western industrial nations.