Sinusitis, also known as rhinosinusitis, is inflammation of the mucous membranes that line the sinuses resulting in symptoms that may include thick nasal mucus, a plugged nose, and facial pain. Other signs and symptoms may include fever, headaches, a poor sense of smell, sore throat, and a cough. It is defined as acute sinusitis if it lasts less than 4 weeks, and as chronic sinusitis, if it lasts for more than 12 weeks.

Sinusitis can be caused by infection, allergies, air pollution, or structural problems in the nose. Most cases are caused by a viral infection. Recurrent episodes are more likely in persons with asthma, cystic fibrosis, and poor immune function. X-rays are not usually needed unless complications are suspected. In chronic cases, confirmatory testing is recommended by either direct visualization or computed tomography.