Functional endoscopic sinus surgery has become an increasingly popular treatment for chronic sinusitis. This approach is aimed at re-establishment of ventilation and mucociliary clearance of the sinuses. However, some otolaryngologists believe that the Caldwell-Luc procedure should be routinely used for unilateral chronic sinusitis, because it is often associated with the maxillary sinus carcinomas. To evaluate the state of endoscopic sinus surgery for the diagnosis and treatment of unilateral chronic sinusitis, we analyzed the cases of 39 patients with unilateral chronic sinusitis who underwent endoscopic sinus procedures. These patients were unresponsive to appropriate antibiotic management for more than 6 months. Generally, endoscopic ethmoidectomy and antrostomy were performed with preservation of the middle turbinate. After the ostium was enlarged, the maxillary sinus was cleaned and carefully inspected for the presence of associated neoplasms using 30 and 70 degree endoscopes. Preoperative computed tomography (CT), postoperatve pathologic diagnosis, fiberscopic findings of the maxillary sinus, and symptomatic improvement were evaluated. Three patients had CT evidence of bone destruction of the lateral nasal wall. Pathological diagnosis demonstrated that three patients had maxillary sinus mycoses caused by Aspergillus species, one patient had inverted papilloma, and the other 35 patients had chronic sinusits. No associated malignancy was found. Eighty-one percent of the patients had almost normal endoscopic findings of the maxillary sinus by postoperative fiberscopic examination 4 to 8 months following surgery. With an average follow-up of 26 months, 88% of the patients were judged as having significantly improved in their presenting complaints of mucopurulent rhinorrhea, nasal obstruction, and facial pain. The results of this series suggest that endoscopic sinus surgery is an effective procedure for the diagnosis and treatment of unilateral chronic sinusitis.