BACKGROUND: This article reviews causative factors for asthma and allergy during childhood and describes current trends in prevalence and hospitalisation for childhood asthma. MATERIALS AND METHODS: A review of the prevalence of childhood asthma in Norway over the last 50 years together with recent trends in hospitalisation for childhood asthma, is given. Possible causative factors for childhood asthma are reviewed, and recent theories for the current increase in prevalence are discussed. RESULTS AND INTERPRETATION: During the last 50 years, a steady increase in the prevalence of childhood asthma has been documented through published studies from 1948 and onwards. From 1980 and until 1990, an increase in hospital admissions due to acute asthma was observed; later, admissions have leveled off, particularly as regards readmissions. Smoking during pregnancy and childhood decreases lung function. Allergic sensitisation is related to asthma development and may occur already during pregnancy. Increased allergic sensitisation may occur due to reduced load of infections. However, respiratory virus infections, and especially RS virus and rhinovirus infections, are closely related to asthma development and symptoms during childhood. A reduction in readmissions for asthma may be related to increased use of antiinflammatory therapy for asthma.