Acute sinusitis affects millions of persons in the United States every year and is among the most common reasons for physician visits, prompting over 3 million visits annually (1). The more accurate term for this condition is acute rhinosinusitis, because symptoms involve both the nasal cavity and the sinuses. For simplicity, this review uses the term "sinusitis" for rhinosinusitis. There are 4 pairs of air-filled paranasal sinuses: the frontal, maxillary, ethmoid, and sphenoid sinuses. Acute sinusitis typically occurs in the maxillary sinuses (Figure). Sinusitis is characterized as acute when the duration of symptoms is shorter than 4 weeks, subacute when the duration is from 4 weeks to 12 weeks, and chronic when the duration is more than 12 weeks. Sinusitis seems to be due to congestion and blockage of the nasal passages, usually in response to viral infection or allergic rhinitis but occasionally to other stimuli. The paranasal sinuses become inflamed, and mucus cannot drain properly, providing an environment where bacteria, or rarely fungus, can thrive. Persons with chronic nasal congestion, and particularly those with allergies and asthma, may be more prone to developing acute sinusitis, but it can affect anyone. Suggestive symptoms include headache, congestion, facial pain, fatigue, and cough, all of which can be disruptive to usual activities but are rarely severe.