Eosinophil inflammation is essential in many cases of allergic and non-allergic rhinitis. Activated eosinophils release toxic granule proteins. In this study, we compared the degree of local nasal and systemic eosinophil activation by the determination of eosinophil cationic protein (ECP) in serum and native nasal fluid from 119 patients. We found no significant differences in serum ECP levels of the various patient groups. In all patient groups, except in the vasomotor rhinitis group, nasal fluid ECP levels differed significantly from normal controls. We found a nasal fluid ECP (mean ± SEM) of 32·6 ± 81 ng/ml for normals, 106 ± 39·7 for non-rhinitic atopics, 87·6 ± 20·8 ng/ml for patients with chronic non-allergic sinusitis, 101·3 ± 40·4 ng/ml for patients with a history of pollinosis, 150·5 ± 35·1 ng/ml for patients with acute pollinosis, 84·7 ± 24·7 ng/ml for individuals with perennial allergic rhinitis and 112·9 ± 25·6 ng/ml for patients with both perennial and seasonal allergy. Patients with nasal polyps had mean nasal ECP levels of 146·9 ± 57·7 ng/ml in absence of allergy and 147·9 ± 54·9 ng/ ml in the presence of allergy. Nasal ECP was 67·0 ± 22·4 for patients with hyperreactive rhinitis. We found a significant correlation of 0·95 between nasal eosinophils and nasal ECP. Nasal ECP and a subjective symptom score only correlate significantly for chronic sinusitis. We conclude that monitoring native nasal fluid ECP levels may be useful in the diagnosis and mangement of nasal inflammation. Elevated ECP in nasal secretion may originate from upregulated eosinophil degranulation and thus is a marker for local inflammation although not specific for any particular nasal disease.