We report the case of a young patient who developed spontaneous compressive orbital emphysema after an attack of coughing. At admission the patient presented left proptosis, diplopia, vision impairment and headache. Computer tomography showed air in the lateral part of left orbit compressing the eyeball and the optic nerve medially. It also revealed a sphenoid bone dysplasia with hyperpneumatization of the left greater wing and with two dehiscences in its wall. It was very intriguing to discover that this sphenoid dysplasia and the flap of mucosa covering one dehiscence were causing a ball-valve effect, allowing air to enter but not leave the orbit. Endoscopic sinus surgery was successfully used to treat this case.