The relationship between allergen exposure, sensitization, and the development of asthma is complex. Lowe and colleagues investigated the effect of pet ownership and exposure to cat, dog, and house dust mite allergens on lung function in preschool children at age 3 years. The authors found that pet ownership, sensitization to allergens without exposure, or exposure to allergens in nonsensitized individuals had no effect on lung function. However, the combination of specific sensitization and exposure to a sensitizing allergen was associated with poorer lung function in early life. This study helps to explain the often confusing association between atopy and asthma.