The complement system is an important part of our immune system, and complement defects lead generally to increased susceptibility to infections and autoimmune diseases. We have studied the role of complement activity in relation with chronic rhinosinusitis (CRS), and more specifically studied whether complement defects collectively predispose individuals for CRS or affect CRS severity. The participants comprised 87 CRS patients randomly selected from the general population, and a control group of 150 healthy blood donors. The CRS patients were diagnosed according to the European Position Paper on Rhinosinusitis and nasal Polyps criteria, and severity was evaluated by the Sino-nasal Outcome Test-22. Serum samples were analysed by ELISA for activity of the respective pathways of complement, and subsequently for serum levels of relevant components. We found that the frequency of complement defects was significantly higher among CRS patients than among healthy control subjects. A majority of Mannan-binding lectin deficient CRS patients was observed. The presence of complement defects had no influence on the severity of subjective symptoms. Our studies show that defects in the complement system collectively may play an immunological role related to the development of CRS. However, an association between severity of symptoms and presence of complement defects could not be demonstrated.