BACKGROUND: The observation of an increased prevalence of allergic disorders coinciding with a decreasing frequency of infectious diseases in early childhood has led to the speculation that infections may prevent allergic sensitization. Information on the role of parasites in this context is limited. Bronchiolitis in infancy has been linked with asthmatic symptoms later in childhood, although the underlying cause of this association is unknown. METHODS: To test the hypothesis that early parasitic infections in infancy might prevent the development of allergic manifestations later in life, the effect of malaria infections during the first year of life on the risk of bronchiolitis was studied in 675 Tanzanian children at 18 months of age. The study was conducted as part of an intervention trial of malaria chemoprophylaxis and/or iron supplementation for the prevention of malaria and anemia in infants. RESULTS: The incidence of bronchiolitis up to 18 months of age in the 675 children was 0.58 episode per child per year. The risk factors analysis was based on 470 children with complete data. There was no difference in the incidence of bronchiolitis between those who had received malaria chemoprophylaxis during the first year of life and those who had not. However, the proportion of children who had bronchiolitis was lower among those who had had malaria episodes than among those who had not (48% vs. 55%, P = 0.05). CONCLUSIONS: This study does not support the hypothesis that reduced exposure to parasites may modulate the development of bronchiolitis early in life.