The complications of endoscopic sinus surgery performed on 307 patients with chronic inflammatory paranasal sinus diseases were analysed. The results showed that: 1. The common complications were excessive bleeding, orbit penetration, intra-orbital haemorrhage, postoperative stenosis of the maxillary ostium and middle turbinate adhesions. 2. The most common perioperative complications were those related to orbital penetration except excessive bleeding. A major complication was a case of cerebrospinal rhinorrhea with secondary purulent meningitis. 3. The annual overall incidences and the rates of common complications have been significantly decreased with year. The causes and prevention of excessive bleeding, stenosis of maxillary ostium and middle turbinate adhesions are discussed emphatically. In addition, the proposals were offered concerning the diagnostic criteria of the complications, and the statistical method of the complication incidence.