The risk factors for sensitization to pets was investigated in 169 male pupils. A recent or former contact with cats, dogs, and guinea pigs in own home was reported in 52 (30.8%), 42 (24.9%), and 20 (11.8%) study subjects, respectively. Clinically manifested allergy was found in two probands to cats and in two others to guinea pigs, three of them had formerly had pets and one proband with allergic rhinitis to cats had never had any pet. Sensitization to animals and aeroallergens was investigated with Phadezym-RAST. Only owners of cats had a higher incidence of cat sensitization than probands without direct contact (26.9% versus 10.3%, P less than 0.01). No statistically significant difference in sensitization to dogs and guinea pigs was found in groups with and without these pets. A strong correlation existed between sensitization to pets and other aeroallergens (house dust mite: P less than 0.025, birch pollen: P less than 0.0001, mugwort: P less than 0.0001, and grass pollens: P less than 0.0001). No association was found between sensitization to pets and smoking history, bronchial hyperreactivity to methacholine or radiological findings of the paranasal sinus.