BACKGROUND: Causes of transient vertical diplopia include myasthenia, dysthyroid orbitopathy, multiple sclerosis, decompensation of a pre-existing state. PURPOSE: To describe an unusual cause of transient diplopia, silent sinus syndrome. METHODS: Clinical, radiological and pathology findings from a 55-year-old man with acquired transient vertical diplopia are reported. RESULTS: Eight months after onset of symptoms, vertical diplopia was constant. Hypoglobus and enophthalmos were present. On CT scan, orbital floor was thinned and lowered by 8 mm. Maxillary sinus was filled with material compatible with the histological diagnosis of mucocele. Sinus surgery and orbital floor reconstruction resulted in resolution of diplopia. CONCLUSION: Sinus disorders can produce diplopia and/or visual loss, when invading the orbit. Usually, an acute orbitopathy is obvious. Our case shows that chronic sinus disorders can produce "silent" oculomotor disturbances and, at least in the beginning, transient diplopia.