We looked for an association between early exposure to pets and asthma and wheezing in children whose mothers or fathers did or did not have a history of asthma. We followed up 448 children, who had at least one parent with a history of atopy, from birth to 5 years. Among children whose mothers had no history of asthma, exposure to a cat or a Fel d 1 concentration of at least 8 microg/g at the age of 2-3 months was associated with a reduced risk of wheezing between the ages of 1 and 5 years. However, among children whose mothers did have a history of asthma, such exposures were associated with an increased risk of wheezing at or after the age of 3 years. There was no association between wheezing and exposure to dog or dog allergen, and the father's allergy status had no effect on the relation between childhood wheezing and cat exposure.