Rationale: In late October 2003, Southern California wildfires burned more than 3,000 km². The wildfires produced heavy smoke that affected several communities participating in the University of Southern California Children's Health Study (CHS).

Objectives: To study the acute effects of fire smoke on the health of CHS participants.

Methods: A questionnaire was used to assess smoke exposure and occurrence of symptoms among CHS high-school students (n = 873; age, 17–18 yr) and elementary-school children (n = 5,551; age, 6–7 yr), in a total of 16 communities. Estimates of particulate matter (PM₁₀) concentrations during the 5 d with the highest fire activity were used to characterize community smoke level.

Main Results: All symptoms (nose, eyes, and throat irritations; cough; bronchitis; cold; wheezing; asthma attacks), medication usage, and physician visits were associated with individually reported exposure differences within communities. Risks increased monotonically with the number of reported smoky days. For most outcomes, reporting rates between communities were also associated with the fire-related PM₁₀ levels. Associations tended to be strongest among those without asthma. Individuals with asthma were more likely to take preventive action, such as wearing masks or staying indoors during the fire.

Conclusions: Exposure to wildfire smoke was associated with increased eye and respiratory symptoms, medication use, and physician visits.