The objective of this cross-sectional study was to compare the prevalence of bronchial asthma among Saudi schoolboys in Yanbu Industrial City and in two non-industrial villages. In 1993, questionnaires were distributed to 375 schoolboys for completion by their parents. The prevalence of questionnaire-diagnosed asthma in Yanbu Industrial City, and in the villages of Al-Furash and Al-Gafure, was 12.6%, 4.3% and 16% respectively. The prevalence of physician-diagnosed asthma in the three areas was 13.9%, 2.2% and 13.7% respectively. There was no significant difference between the two methods of diagnosis.