The study investigates the association between outdoor air pollution and chronic rhinosinusitis (CRS) in Cologne, Germany. First, using their addresses, all patients treated for CRS at the ENT Department of the University Hospital between 1990 and 1999 were assigned to one of the 85 city districts. Second, indicators of air pollution (SO(2), NO(x), TSP) were linked to these areas. Third, to control for socioeconomic confounding, data reflecting the socioeconomic and demographic composition of these districts were collected. Regression analyses reveal weak but consistent statistical effects of pollution on the prevalence of CRS in those parts of the city with air pollution levels above average.