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Objectives: Among Mexican Americans in the United States, children who were born in the US had higher rates of asthma than their Mexico-born peers. The purpose of this study was to examine the associations of doctor-diagnosed asthma with immigration-related variables and to investigate whether these associations could be explained by factors that may change with migration., Methods: We surveyed parents of 2,023 school children of Mexican descent and examined the associations of asthma with nativity, age at immigration, and length of residence in the US after adjusting for potential confounding variables., Results: In multivariate analyses, US-born children had a 2.42-fold (95% confidence interval [CI]: 1.52-3.83) increased odds of asthma compared with their Mexico-born peers. Mexico-born participants who moved to the US before 2 years of age were almost twice as likely to experience asthma compared with Mexico-born children who moved to the US >or=2 years of age. In addition, Mexico-born participants who lived in the US for 10 years or more were 2.37 times more likely to have asthma than Mexico-born students who lived in the US for less than 10 years. These associations were not explained by a wide variety of factors such as place of residence in infancy; exposure to animals/pets; history of infections, Tylenol use, and antibiotic use in infancy; breastfeeding; exposure to environmental tobacco smoke; daycare attendance and number of siblings; and language use., Conclusions: Our findings point to the effects of nativity, age at immigration, and duration of residence in the US on the risk of asthma in Mexican American children, suggesting that potentially modifiable factors that change with migration may be linked with the disease. The findings of this study should stimulate further research to explain factors that may be responsible for the observed differentials in the risk of asthma among Mexican Americans.