We discuss the results of endoscopic endonasal sinus surgery for chronic pan-sinusitis characterized by nasal polyposis, especially the effects of post-operative long-term administration of low-dose erythromycin (EM) therapy. The subjects analysed in this retrospective study are surgical cases who had initially been operated for chronic pan-sinusitis. They are classified into one group who has received a post-operative long-term, low-dose EM regimen and into another group who has not received this treatment. The groups have been compared with respect to: (1) the degree of improvement in the post-operative subjective symptoms; (2) postoperative objective findings of the ethmoidal sinus and the ostium of the frontal sinus; and (3) the degree of improvement in the maxillary sinus lesion. Better improvement is achieved in subjective symptoms and objective findings in the EM group than in the non-EM group.