A valid measure of rhinosinusitis health status and quality of life is required for the complete assessment of treatment effectiveness. The purpose of this study was to analyze the psychometric and clinimetric validity of the 20-Item Sino-Nasal Outcome Test (SNOT-20), a disease-specific, health-related quality-of-life measure for rhinosinusitis. The SNOT-20 is a modification of the 31-Item Rhinosinusitis Outcome Measure, and it contains 20 nose, sinus, and general items. To complete the instrument, patients indicate how much they are affected in each area and identify the 5 most important items. The SNOT-20 was completed by 102, 72, and 46 patients at the initial visit and at 6 months and 1 year after treatment commencement, respectively. Cronbach's alpha was 0.9; test-retest scores were highly correlated (r = 0.9). Patients who were more affected had greater SNOT-20 scores (P < 0.002), and patients who had improved had greater change scores (P < 0.04). Items identified as important had greater scores (P < 0.0001) and showed greater change scores (P < 0.0002). The SNOT-20 is a valid outcome measure for patients with rhinosinusitis; it describes the health burden and is sensitive to clinical change.