It has been claimed that parasitic infestation suppresses the development of asthma. Eight Tanzanian schoolchildren in whom asthma was confirmed by the demonstration of exercise-induced bronchospasm were found in an ascaris-endemic area of rural Tanzania. Faecal parasites were found equally commonly in the children with asthma and the ninety-seven non-asthmatic controls. The prevalence of asthma (3.3%) in the school under study was comparable with that found in developed countries.