In sensitized subjects with allergic rhinitis (AR) or asthma, allergen exposure provokes symptoms. Among non-asthmatics with AR, an association between allergen sensitization, pollen season and lower airway inflammation has been demonstrated. Our aims were to compare AR and asthma with regard to patterns of allergen sensitization, the degree of airway hyperresponsiveness (AHR) and levels of exhaled nitric oxide (ENO). Finally, we wanted to relate our findings to previous or current exposure to household pets. Four hundred and thirty-one adolescents with different clinical phenotypes were randomly selected from a large-scale epidemiological survey. They were investigated with allergy screening, measurements of ENO and a methacholine bronchoprovocation test. Sensitization to pet allergens (cat, dog and horse) was associated with increased AHR and ENO both in asthmatics and non-asthmatics with AR. The risk of being sensitized to cat allergens was significantly reduced in those who had kept cats vs. those who had never kept them. Keeping dogs or horses did not influence the risk of being sensitized to the respective allergens. Only in steroid-naive, non-smoking asthmatics, a trend towards increased ENO in those sensitized and exposed to cat or dog allergens was seen. Although sensitization towards pet allergens was associated with inflammation in the lower airways irrespective of clinical phenotype, keeping pets did not increase the risk of being sensitized to pet allergens.