Hydatid cyst: an unusual cause of diplopia. 
A 59-year-old man presented with double vision due to vertical displacement of one eye by an infraorbital mass that indented the globe.  Some 10 years previously he had had a Caldwell-Luc antrostomy for chronic sinusitis.  Following investigation, the antrostomy was revised and an orbital cyst with a histological appearance suggestive of a mucocele was excised via an anterior inferior orbitotomy.  The orbital mass recurred with identical symptoms some 18 months later with a second mass confluent with the first, presenting subcutaneously through his zygoma.  A further radical antrostomy was performed enabling a diagnosis of a hydatid cyst of the antrum to be made.  The unusual features of this rare cause of diplopia are discussed and illustrated.  The literature on orbital hydatid cysts is reviewed and recommendations made as regards adequate surgical exposure and not irrigating orbital cysts with noxious agents.