To determine if there is an association between low birthweight (< 2,500 g) and asthma. The key words "asthma," "children," "birth weight," and "risk factors" were used to identify analytical epidemiological studies on humans that were published in English, Portuguese, or Spanish between 1990 and 2001 and that were indexed in either of two bibliographic databases: MEDLINE (PubMed) and Latin American and Caribbean Health Sciences (Literatura Latino-Americana e do Caribe em Ciências da Saúde (LILACS)). We identified 41 articles dealing with the association between birthweight and asthma: 21 were longitudinal studies, 16 were cross-sectional studies, and 4 were case-control studies. Low birthweight was identified as a risk factor for asthma in 26 of the articles. Two studies found an inverse association between birthweight and risk for asthma, and 15 did not find any association. The association between low birthweight and asthma remains controversial, mainly due to the extensive network of factors that contribute to the development of asthma. The association between low birthweight and asthma occurs mainly among children up to 5 years of age, and it decreases as age increases. Pulmonary function is an important factor that needs to be considered. Low birthweight children may have decreased pulmonary function and consequently develop asthma. On the other hand, these children may have asthma as a consequence of low birthweight, without pulmonary function being a factor. In the six studies comparing low birthweight children (< 2,500 g) with very low birthweight children (< 1,500 g), very low birthweight was associated with a greater risk for developing asthma. Finally, the contribution of intrauterine factors (such as nutrition) in the development of asthma is receiving increasing attention in the scientific literature. Controlling for possible confounding factors is essential to clarifying the relationship between birthweight and asthma.