The relationships between mite and cat allergen levels in the home, skin test reactivity, and severity of asthma were explored in 120 asthmatic subjects, 57 in Vancouver and 63 in Winnipeg. Patients in the two cities were similar in age, gender distribution, and severity of asthma. Skin tests were performed using 13 common allergens, including D. pteronyssinus, D. farinae, and cat dander, as well as control subjects. Patients recorded their asthma symptoms, medication requirement, and twice daily peak expiratory flow rate for 1 yr. Dust samples were collected every 3 mo during each season of 1992 through 1993 in both cities. Mite and cat allergen levels were determined with an ELISA using monoclonal antibodies against Der p I, Der f I, and Fel d I. There was no relationship between skin test reactivity and levels of mite and cat allergens. In children with positive skin tests to either mite allergen, total mite (sum of Der p I and Der f I) allergen level was positively related to the mean daily symptom score and negatively related to the daily mean PEF (% of predicted). There was no such relationship among adult asthmatic patients with positive skin tests to either mite allergen. No relationship was found between cat allergen level and the severity of asthma.