OBJECTIVES: The epidemiology of acute sinusitis and acute exacerbation of chronic sinusitis (AECS) is still unknown in Germany. This study aims to make an estimate of the prevalence of acute sinusitis and AECS in Germany. METHODS: A representative sample of 63 GPs and 40 ear, nose and throat specialists have been asked to document all patients older than 16 years with acute sinusitis or AECS appearing in their practice over a period of one week (during the time frame from February 2nd to April 1st, 2001). Afterwards, an expansion for Germany has been calculated /estimated. RESULTS: Overall, 2250 patients suffering from acute sinusitis or AECS has been documented in 8 weeks. One thousand one hundred eighty-seven patients visited a GP and 1063 patients visited an ear, nose and throat specialist. Eighty percent of patients suffered from acute sinusitis, whilst 20% had an acute exacerbation of chronic sinusitis. On the average, each GP treated 19 sinusitis patients per week, i.e. 9% of total patients visiting the practice. An ear, nose and throat specialist treated 27 patients per week which means 11% of his total patients. Based on the assumption that 67.5 million people older than 16 years live in Germany (2001), there are approximately 1.215.000 sinusitis patients per week, i.e. 1.8% of German population having a sinusitis and seeing their GP in an average winter week. CONCLUSIONS: Assuming at least 20 weeks of wintertime (from November to March), there are approximately 24.300.000 sinusitis patients in Germany within this time frame. This means hardly one third of German population. When interpreting this result it is important to take into account that sinusitis is a disease with repeating exacerbations.