A method is presented for the objective monitoring of the inflammatory response of the nasal mucosa to natural allergen exposure through measurements of biochemical markers such as TAME-esterase activity in a daily nasal lavage. Nine patients with strictly seasonal allergic rhinitis caused by birch pollen and five healthy nonatopic control subjects participated in the present study, which started 1 wk before the birch pollen season and continued throughout the entire pollen season. A diary card was used to assess daily nasal symptoms, and a daily nasal lavage was performed once every afternoon. The degree of pollen exposure was assessed by daily pollen counts. The TAME-esterase activity in the lavage fluid from the allergic patients, but not in that from the nonallergic control subjects, increased significantly during pollen exposure despite low pollen counts and was found to correlate with daily nasal symptoms (r = 0.36; p less than 0.05) and the degree of pollen exposure (r = 0.45; p less than 0.01). The area under the curve for the daily TAME-esterase measurements thus differed significantly (p less than 0.02) between allergic patients (158 +/- 28) and control subjects (74 +/- 9). This approach for the monitoring of patients with hay fever during natural allergen exposure will make further objective studies on the pathophysiology and pharmacology of hay fever possible. We also provide support for the validity of previous findings obtained in allergen challenge situations utilizing the lavage approach.