The prevalence of asthma among 10,527 children aged 7-16 years was investigated in 1985. Children were assigned to the asthma group when replies to a questionnaire stated that at least two of four stipulated symptoms had been experienced in the preceding year. A validation study, comprising exploration of medical history, pulmonary auscultation and physiological tests (spirometry and methacholine inhalation challenge) was performed in 73 children. The overall prevalence of asthma was 4%. The most common symptom-inducing factors were physical exertion, upper respiratory tract infection, contact with animals and tobacco smoke. The methacholine test had limited value as a diagnostic aid, being positive in only 25% of children with a clear history of asthma.