The determinants of variability in the clinical expression of atopy are still to be documented. The goals of this study were to determine, in subjects with a clinical diagnosis of symptomatic asthma or rhinitis, what is the possible contribution of different types of indoor and outdoor allergens to the development of their disease, by looking at the prevalence and degree of sensitization to these allergens according to age and gender.We analysed allergy skin prick tests to common airborne indoor and outdoor allergens in 3371 consecutive patients, grouped according to diagnosis of allergic asthma, rhinitis, or both. For each of these three groups, we calculated the prevalence of sensitization to indoor/outdoor allergens, the atopic index (AI), the number of positive responses to allergy skin prick test and the mean wheal diameter (MWD) of these responses.The prevalence of atopy and the values of AI and MWD peaked in subjects aged 16 to 25 years, declining afterwards; in subjectsor = 18 years old, atopic indices were slightly higher in men than in women. In atopic subjects, the prevalence of sensitization was, in decreasing order, housedust (84.2%), cat hair-epithelium (76.5%), dog hair-dander (63.0%), house dust mite (54.2%), grasses (51.9%), trees (47.2%) and ragweed pollens (44.9%) and finally, moulds (25.4%). Among subjects sensitized only to outdoor allergens (n = 195). 73.8% had a rhinitis, 11.8% had asthma and 14.4% had both diagnoses; for those sensitized only to indoor allergens (n = 710), these values were respectively 48.6, 24.5 and 26.9%, and for those sensitized to both indoor and outdoor allergens (n = 1793), the comparable values were 55.5, 14.6 and 29.9%.These data show that in our population of subjects with respiratory allergic symptoms, indoor allergen sensitization is strongly associated with asthma, while exclusive sensitization to pollens is associated primarily with rhinitis. Sensitization was more prevalent for indoor allergens than for outdoor allergens in all groups determined according to diagnosis or age. Indices of atopy were higher in men in the groupor = 18 years old. Prevalence and degree of sensitization were shown to peak in young adults, regardless of the allergen, and to diminish with age. This study stresses the role of indoor allergens in the development of asthma and shows the variability of allergic manifestations according to the type of sensitization.