Allergic disease affects millions of persons in the United States. Environmental control measures are essential for persons who are sensitive to dust mite and cat allergens and may be important in persons who are sensitive to cockroaches and fungus. The bedroom is the most important room in which to implement environmental control measures, such as encasing mattresses and pillows in vinyl or semipermeable covers. Patients with asthma (especially persistent asthma) should be considered for allergy testing and more aggressive environmental control measures. Carpet in the bedroom or over concrete should be removed. For persons who are sensitive to pet allergens, permanent removal of the pet is recommended. Dust mite and fungus growth can be controlled by keeping the household humidity level at less than 50 percent. Removing sources of food and water and using insecticides will help control cockroach populations. Patients who are sensitive to mold and outdoor air pollution should consider limiting outdoor activities when these levels are high. Patients with asthma should avoid exposure to tobacco smoke. (Am Fam Physician 2002;66:429-30.)