Objectives: Chronic rhinosinusitis may be accompanied by impaired immunity despite normal levels of serum immunoglobulins. Immune responses in sinusitis patients have previously been evaluated using polysaccharide vaccines. Our AIM was to assess the immune status by evaluating responses to diphtheria and tetanus vaccine. Methods: Specific antibodies were measured before and 2 weeks after vaccination in 25 patients with chronic or recurrent sinusitis and in 30 healthy individuals. The mean age of the patients was 46 years and that of healthy controls 43 years.Results: After vaccination the patients had on average 4.08-fold lower responses to diphtheria toxoid and 2.20-fold lower responses to tetanus than the controls. Fourteen out of 25 patients had antibody levels that did not reach the 95% normal distribution range of healthy controls after either diphtheria or tetanus vaccination. All the patients had normal levels of serum immunoglobulins.Conclusions: A significant proportion of patients with persisting symptoms of rhinosinisitis may have impaired responses to protein vaccines. Responses to protein vaccines may be used to evaluate immune function of sinusitis patients.