Pneumosinus dilatans consists of an abnormal dilatation of the paranasal air sinuses which contain air only and are lined by normal epithelium. The condition is most commonly observed when it affects the sphenoid sinus as a response to a local meningioma of the tuberculum sellae or planum sphenoidale. Pneumosinus dilatans affecting the walls of the paranasal sinuses which form the boundaries of the orbit is less common; six patients reported here presented with either unilateral or bilateral exophthalmos. The frontal sinus and ethmoid cells were affected unilaterally in four patients and the maxillary antra bilaterally in two. The condition was associated with a meningioma in three patients and fibro-osseous disease in three patients. The importance of recognising this condition is in alerting the radiologist to the possible presence of an occult meningioma requiring soft-tissue imaging techniques (computed tomography or nuclear magnetic resonance imaging). The presence of fibro-osseous disease can usually be recognised by plain radiography.