Acute inflammation of sinus mucosa is called acute sinusitis. .The sinus most commonly involved is the maxillary followed in turn by ethmoid, frontal and sphenoid. Very often, more than one sinus is infected (multisinusitis). Sometimes, all the sinuses of one or both sides are involved simultaneously (pansinusitis unilateral or bilateral).

A sinusitis may be 'open' or 'closed' type depending on whether the inflammatory products of sinus cavity can drain freely into the nasal cavity through the natural ostia or not. A 'closed' sinusitis causes more severe symptoms and is also likely to cause complications.