Introduction There are conflicting data on the association between early exposure to pets and allergic diseases. Bias related to retrospective information on pet ownership has been addressed as a reason for distorted study results.

Objective To elucidate how early exposure to cat and dog relates to IgE‐sensitization and asthma in children at 2 and 4 years of age, in a prospective birth‐cohort study.

Methods Four thousand and eighty‐nine families with children born 1994–1996 in predefined areas of Stockholm answered questionnaires on environmental factors and symptoms of allergic disease at birth, one, two and four years of age. Dust samples collected from the mothers' beds at birth were analysed for Fel d 1 and Can f 1 in a subgroup of the cohort. Blood samples taken at four years from 2614 children were analysed for allergen‐specific IgE to common airborne allergens. Risk associations were calculated with a multiple logistic regression model, with adjustment for potential confounders.

Results A correlation was seen between allergen levels and reported exposure to cat and dog. Exposure to cat seemed to increase the risk of cat sensitization, OR (odds ratio) 1.44 (95% confidence interval 1.03–2.01), whereas dog exposure did not have any effect on dog sensitization, OR 1.16 (0.79–1.72). Dog ownership was related to a reduced risk of sensitization to other airborne allergens, OR 0.36 (0.15–0.83), and a similar tendency was seen for cat ownership OR 0.63 (0.37–1.07). Early dog ownership seemed to be associated with a lower risk of asthma, OR 0.50 (0.24–1.03), with no corresponding effect after cat ownership, OR 0.88 (0.56–1.38).

Conclusion Early exposure to cat seems to increase the risk of sensitization to cat but not of asthma at 4 years of age. Dog ownership, on the other hand, appears to be associated with lowered risk of sensitization to airborne allergens and asthma. Both aetiological relationships and selection effects have to be considered in the interpretation of these findings.