Sinusitis, an inflammatory disease of the sinus, is one of the most commonly reported diseases in the United States, affecting an estimated 14% of the population. The prevalence of sinusitis is rising. Between 1990 and 1992, persons with sinusitis reported approximately 73 million restricted activity days—an increase from the 50 million restricted activity days reported between 1986 and 1988. Because critical questions remain unanswered about its cause, pathophysiology, and optimal treatment, sinusitis continues to generate significant health care costs and affects the quality of life of a large segment of the U.S. population. To identify critical directions for research on sinus disease, the American Academy of Allergy, Asthma and Immunology and the American Academy of Otolaryngology-Head and Neck Surgery Foundation, Inc., convened a meeting in January 1996 in collaboration with the National Institutes of Allergy and Infectious Disease. This document summarizes the proceedings of that meeting and presents what is intended to be the background for future investigation of the many unanswered questions related to sinusitis.