Allergy and sinusitis. Nasal allergy is statistically related to inflammatory chronic sinusitis as a risk factor. But one question still remains unanswered: are the reactions and modifications observed in the sinuses after natural exposure to a nasal allergen or after nasal allergen challenge linked to an IgE mediated mechanism? Similarities in symptoms, eosinophils and mediators of inflammation in the mucosa have been found between allergic rhinitis and sinusitis. The same applies for the deposition of Major Basic Protein (MBP) and treatment results, especially when topical steroids are found. An original prospective study was held among 106 patients (24 patients allergic to perennial allergens, 82 non allergic patients) suffering from bilateral chronic inflammatory (no polyposis) ethmoidal sinusitis. The allergic group was submitted to a 3 months antiallergic treatment (Cetrizine 10 mg once a day, Beclomethasone dipropionate 50 μg three times a day) before being referred for bilateral endonasal ethmoidectomy under endoscopic control. Scores for rhinorrhea, nasal obstruction and global comfort (global assessment) were compared before and after ethmoidectomy. Both groups were significantly improved by surgery. Comparing both groups, no significant difference was found before and after surgery regarding the three above mentioned parameters. This suggests that 1) symptoms are common to both perennial nasal allergy and chronic ethmoidal sinusitis, 2) medical treatment failure in allergy must require a CT scan of the sinuses to assess a possible accompanying chronic sinusitis, 3) chronic ethmoidal sinusitis is probably the leading factor responsible for nasal symptoms such as rhinorrhea and nasal obstruction when associated with perennial allergy.