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The authors studied children who were 10-12 yr of age and who resided in sister cities in a U.S.-Mexico border region to determine the prevalence of asthma and respiratory symptoms. The relationship of symptoms to ambient levels of particulate matter less than 10 μm in diameter (PM10), and to several indoor environmental conditions, was assessed. The study was conducted in the border cities of Ambos Nogales (Nogales, Arizona [United States], and Nogales, Sonora [Mexico]). At the beginning of the 11-wk study, during the autumn of 1996, 631 students and their parents completed baseline questionnaires. While in school, the children completed daily symptom diaries and daily peak expiratory flow maneuvers. PM10 values and daily temperatures were also measured. The authors found that the prevalence of self-reported asthma among 5th-grade students was comparable on both sides of the border (i.e., 7.6% on the Arizona side and 6.9% on the Sonora side). Wheezing was a frequent complaint (29.5-35.6%), as was cough (16.8-29.6%). Smoking in the home was common on both sides of the border, and it was associated with a greater occurrence of self-reported asthma and respiratory complaints. Increased respiratory symptoms were also associated with increased ambient PM10 levels. The prevalence of respiratory symptoms such as wheezing and frequent cough among all children in this study, combined with the limitations inherent in self-reporting, suggest that asthma may actually be more prevalent than has been previously reported.