We studied hypersensitivity to cats and dogs in three hundred adult asthmatics who kept them. Some findings were noted as follows. 1) The mean duration for which cats were kept was 6.2 +/- 10.1 (1 S.D.) years and that for dogs was 4.2 +/- 9.6 (1 S.D.) years. 2) The positive rate in intradermal tests to dog dander was 7% and to cat hair-epithelium 32%. The positive RAST (score > or = 2) rate to cat epithelium was 36.1%, to dog dander 31.6% and to dog epithelium 7.3% respectively. 3) Eleven to 18% of the dog keepers and 23 to 36% of the cat keepers whose RAST score were positive were aware of the relationship between pets and asthma, specifically concerning eye and nose symptoms. 4) Thirty percent of the cat and dog keepers had become sensitized to them after keeping them for under one year. The positive RAST rate was unchanged in those who kept Japanese native dogs but increased in those who kept foreign breeds after 3 to 6 years. The rate of sensitization rate to cats tended to increase along with the period for which they were kept. 5) Dog dander RAST scores among the foreign breed keepers were significantly higher than those among the native dog keepers. If the dog breed was the same, the RAST score was not higher if the dog was kept indoors than if it was kept outdoors. We concluded that a possible reason why the RAST scores were higher among those who kept foreign breeds was that foreign breeds have stronger allergenicity.