Objective: To determine the prevalence and severity of wheezing in infants, using the standardized protocol devised for the "Estudio Internacional de Sibilancias en Lactantes" (EISL, International Study of Wheezing in Infants), as well as to determine the relationship between such wheezing and physician-diagnosed asthma, in the first year of life. Methods: Between March of 2005 and August of 2006, the EISL questionnaire was administered to the parents or legal guardians of infants undergoing routine procedures or immunization at public primary health care clinics in the southern part of the city of Sao Paulo, Brazil. Results: Our sample comprised 1,014 infants (mean age = 5.0 +/- 3.0 months), 467 (46.0%) of whom had at least one wheezing episode, 270 (26.6%) having three or more such episodes, in their first year of life. The use of inhaled beta(2) agonists, inhaled corticosteroids, or antileukotrienes, as well as the occurrence of nocturnal symptoms, difficulty breathing, pneumonia, emergency room visits, and hospitalization due to severe wheezing, was significantly more common among those with recurrent wheezing (p < 0.05). Physician-diagnosed asthma was reported for 35 (7.5%) of the 467 wheezing infants and was found to be associated with the use of inhaled corticosteroids, difficulty breathing during the attacks, and six or more wheezing episodes in the first year of life. However, less than 40% of those infants were treated with inhaled corticosteroids or antileukotrienes. Conclusions: In this study, the prevalence of wheezing episodes among infants in their first year of life was high and had an early onset. The proportion of infants diagnosed with and treated for asthma was low.