In this case-control study, researchers assessed if more severe disease occurs in subjects with asthma that migrated from rural areas to a large urban center in Brazil as compared to asthmatic subjects that was born and always lived in the urban area. The asthmatics currently residing in a large urban center in Brazil were analyzed, including 486 subjects with moderate-severe asthma and 432 subjects with mild asthma. Compared to urban-born subjects, more severe asthma was found in rural-urban migrants in Brazil. A high burden of asthma in the urban area was reported in relation to rural-urban migration.