All patients will be seen in the outpatient setting. After skin testing, subjects meeting established criteria will be placed in the study. Subjects will use dust mite Dermatophagoides pteronyssinus extract or placebo nasal spray at home for 2 weeks, with a 1 month washout period followed by 2 weeks of the other nasal spray. Both patients and investigators will be blinded as to which spray is being used. Subjects will monitor symptoms using a symptom score card and return to the research unit after the first and second week of challenge for review of symptoms and clinical assessment. If symptoms increase sufficiently to make the subject uncomfortable, they will return earlier. Testing will consist of peak nasal inspiratory flow rate and nasal lavage for PGD2 and ECP. Subjects will have these tests performed at the beginning of the study, and at the end of weeks 1 and 2.