OBJECTIVE: The objective of this study was to determine the rate of complications of surgery for nasal polyposis and chronic rhinosinusitis as well as their risk factors. STUDY DESIGN, SETTING, PARTICIPANTS, AND OUTCOME MEASURES: The authors conducted a prospective study of 3,128 patients who underwent sinonasal surgery during 2000 and 2001 in 87 National Health Service hospitals in England and Wales. Patients completed a preoperative questionnaire that included the Sino-Nasal Outcome Test, a measure of sinonasal symptoms severity and health-related quality of life. Surgeons provided information about polyp extent, opacity of the sinuses on computed tomography (Lund-Mackay score), comorbidity (American Society of Anesthesiologists score), and the occurrence of perioperative complications. RESULTS: Major complications (orbital or intracranial complications, bleeding requiring ligation or orbital decompression, or return to the operating room) occurred in 11 patients (0.4%). Minor complications (all other untoward events) occurred in 207 patients (6.6%). Most frequently reported minor complications were excessive perioperative hemorrhage bleeding (5.0%) as well as postoperative hemorrhage requiring treatment (0.8%). Multivariate analysis indicated that the complication rate was linked to the extent of disease measured in terms of symptom severity and health-related quality of life, the extent of polyposis, level of opacity of the sinuses on computed tomography, and the presence of comorbidity, but not surgical characteristics (extent of surgery, use of endoscope or microdebrider, grade of surgeon, and adjunctive turbinate surgery). CONCLUSIONS: The risk of complications depended on patient characteristics rather than on the surgical technique used. Measures of the extent of disease and comorbidity may help in identifying patients at high risk of complications.