Underdiagnosis and undertreatment of chronic sinusitis in children. A review of outpatients diagnosed with sinusitis during the previous year by the allergy services of Childrens Hospital of Los Angeles and Kaiser Permanente, Orange County, revealed 34 patients who had protracted discomforting and debilitating symptoms. The mean duration of symptoms was approximately six months (range 1-14 months). None had been previously diagnosed as having sinusitis or received appropriate treatment. Although all were referred for evaluation of allergic rhinitis and/or asthma, approximately 1/3 were found to have neither of these conditions. Among patients who had asthma, their asthmatic symptoms were aggravated by the sinusitis. All patients eventually responded favorably to appropriate antibiotic therapy. Earlier diagnosis and appropriate therapy would have foreshortened the course of this very discomforting condition. In recent years, we have observed a marked increase in the number of patients referred with the chief complaint of "worsening allergy" and who were subsequently found to have sinusitis. Most of the patients experienced significant incapacitation for protracted periods before a correct diagnosis was made and treatment instituted. In all but a few cases, prompt and sustained improvement followed appropriate therapy. The purpose of this report is to present a retrospective review of 34 illustrative cases. 