Chronic sinusitis certainly deserves to be included in this issue of Immunology and Allergy Clinics of North America devoted to masqueraders of allergy. Although chronic sinusitis is present in almost 32 million Americans, making it more common than hypertension (27 million) or arthritis (25 million), it remains an underappreciated and underdiagnosed condition. Because chronic sinusitis generally presents in an exceedingly subtle fashion (to be discussed in more detail under the section entitled “Diagnosis: History and Physical Examination” later in this article), the clinician must have an increased diagnostic index of suspicion. Patients frequently attribute their chronic nasal congestion and postnasal drainage to “allergy.” I am continually amazed by the many patients who believe that having yellow to green postnasal drainage is not at all unusual and that nasal obstruction and a purulent postnasal drip are things that one has to “live with.”