BACKGROUND There have been no recent assessments of the economic burden of sinusitis in the peer-reviewed literature. OBJECTIVE We sought to estimate the 1996 total direct health care expenditures for the treatment of sinusitis. METHODS This study determined (1) direct expenditures of medical and surgical encounters in which sinusitis was the primary diagnosis and (2) attributable expenditures when related airway diseases were the primary diagnosis and sinusitis was a comorbid condition. An expert panel used the Delphi consensus-building technique to determine the proportions for the latter. RESULTS Overall health care expenditures attributable to sinusitis in 1996 were estimated at $5.8 billion, of which $1.8 billion (30.6%) was for children 12 years or younger. A primary diagnosis of acute or chronic sinusitis accounted for 58.7% of all expenditures ($3.5 billion). About 12% each of the costs for asthma and chronic otitis media and eustachian tube disorders were attributed to diagnosis and treatment of comorbid sinusitis. Nearly 90% of all expenditures ($5.1 billion) were associated with ambulatory or emergency department services. CONCLUSION The economic burden of sinusitis in the United States is significant. However, the limitations of this type of evaluation suggest the $5.8 billion amount may be an underestimate of the true direct costs.