BACKGROUND The Clara cell 10 (CC10) protein is produced by the airway epithelium. Reduced levels of CC10 are associated with allergic rhinitis and asthma. In experimental models, treatment with the CC10 protein may reduce features of airway inflammation. OBJECTIVE To examine whether or not topical treatment with recombinant human CC10 (rhCC10) affects symptoms and signs of allergic rhinitis in a pollen season model. METHODS Out of the pollen season, patients with allergic rhinitis received treatment with rhCC10, 0.56 mg per nasal cavity, once daily for 7 days in a double-blinded, placebo-controlled, crossover design. During this period, individualized allergen challenges were given once daily. Symptoms and peak nasal inspiratory flow (PNIF) were recorded daily in the morning, 10 minutes after challenge, and in the evening. Mean recordings of the last 3 days of the challenge series were used in the analysis. Nasal lavages were performed at the end of each challenge period, and eosinophil cationic protein, myeloperoxidase, and alpha2-macroglobulin levels were measured as indices of eosinophil and neutrophil activity and plasma exudation, respectively. RESULTS Recombinant human CC10 did not affect allergen-induced morning, postchallenge, or evening symptoms compared with placebo. Morning, postchallenge, and evening PNIF were not improved by rhCC10. No statistically significant differences were observed between rhCC10 and placebo for any of the lavage fluid indices. CONCLUSIONS Repeated nasal administrations of rhCC10 protein, in the present dose, do not exert antiallergic effects in seasonal allergic rhinitis.