Acute bacterial sinusitis usually occurs following an upper respiratory infection that results in obstruction of the osteomeatal complex, impaired mucociliary clearance and overproduction of secretions. The diagnosis is based on the patient's history of a biphasic illness ("double sickening"), purulent rhinorrhea, maxillary toothache, pain on leaning forward, pain with a unilateral prominence and a poor response to decongestant therapy. Radiographs and computed tomographic scans of the sinuses generally are not useful in making the initial diagnosis. Since sinusitis is self-limited in 40 to 50 percent of patients, the expensive, newer-generation antibiotics should not be used as first-line therapy. First-line antibiotics such as amoxicillin or trimethoprim-sulfamethoxazole are as effective in the treatment of sinusitis as the more expensive antibiotics. Little evidence supports the use of adjunctive treatments such as nasal corticosteroids and systemic decongestants. Patients with recurrent or chronic sinusitis require referral to an otolaryngologist for consideration of functional endoscopic sinus surgery.