Alloplastic implants are known to be inert for many years, though complications are infrequently reported many years after their insertion. We report the case of a patient who had undergone a blow-out fracture repair five years before the discovery of a hematic cyst. He had been free of symptoms for the first five years after his orbital floor repair but then developed pain on eyeball movement and persistent vertical diplopia, which finally led to surgical intervention. At surgery, a hematic cyst was found to have formed around the implanted silastic plate. When alloplastic material is used in orbital fracture repair, we should be alert for late complications which may occur many years after surgery.