Precise epidemiological data for the prevalence of childhood asthma were lacking for Switzerland until recently. In 1990 we performed a stratified cluster sampling of schoolchildren (aged 7, 12, and 15 years), using a parent completed questionnaire to obtain data for the 12 months prevalence of asthma symptoms and the lifetime prevalence of asthma diagnosis. A response rate of 97.5% enabled us to analyse 4,353 completed questionnaires. The prevalence of any asthma symptom during the last 12 months was 17.5% while only 4.8% of the children reported the diagnostic label "asthma". The 12 months prevalence of chronic night cough was 12% and is comparable to other European data. Wheeze (5.9%) was reported less often in Switzerland than in England. At the age of 7 years asthma symptoms such as wheeze, morning tightness, and allergen-induced symptoms were reported more often in boys than in girls; at the age of 12 and 15 the male preponderance was no more evident. For all asthma symptoms the male-female ratio decreased with increasing age of the children, while independently of age twice as many boys than girls reported the diagnostic label "asthma." We conclude that asthma symptom prevalence in Swiss schoolchildren is within the lower range of European data. Chronic night cough might be a more appropriate variable to compare prevalence rates between regions with different cultural and linguistic backgrounds that the symptom of wheeze. Evidence exists for a substantial underdiagnosis of bronchial asthma in Swiss children, especially in girls. Further evaluation is needed to define risk factors for underdiagnosis and the associated risk for undertreatment.