Reasons for the gender differences in prevalence rates for asthma remain unclear. We have examined the relationships between allergen skin-test reactions and diagnoses of hay fever and asthma in New Zealand boys and girls examined at the age of 13 years. Information on current and past wheezing, diagnosed asthma, and hay fever was obtained for 662 subjects (341 boys) of a birth cohort followed longitudinally to the age of 13 years, using a physician-administered questionnaire. Atopic status was determined by skin-prick tests to 11 common allergens. The proportion of 13-year-old boys with current asthma was 1.6 times higher and of ever-diagnosed asthma 1.4 times higher than in girls, but the prevalence of recurrent wheeze (> or = three episodes per year) not diagnosed as asthma, or of hay fever, was not significantly different between the sexes. The prevalence of diagnosed asthma increased with increasing numbers of positive skin tests, but hay fever without asthma was little affected above one positive skin-test. Boys had a greater prevalence of any positive skin-test (50.1% vs 37.1%), two or more positive tests (29.3% vs 21.8%), and responses to house dust mite (34.0% vs 23.1%) and cat (14.7% vs 11.2%). Gender differences for asthma became insignificant when adjusted for skin-test responsiveness to house dust mite and/or cat. The proportion of children with diagnosed asthma increased with increasing size of weals to house dust mite and cat dander. Gender differences in allergen sensitivities partly explain the gender differences in diagnosed asthma in children. In both sexes, risk of asthma was primarily associated with sensitization to indoor allergens (house dust mite and cat), and was related to the magnitude of the skin-test response, while the risk of hay fever was primarily associated with grass pollen sensitivity.