A 73-year-old white man presented with painless proptosis of the right eye 18 years after retinal detachment surgery on the same eye. Computed tomography (CT) showed a mass in the area of the lateral rectus muscle and magnetic resonance imaging (MRI) defined the multiloculated cystic mass to be located within the lateral rectus muscle. At the time of surgery, a large, thin-walled cystic lesion was found within the lateral rectus muscle. Histopathologically, the cystic mass was of nonkeratinizing stratified squamous conjunctival epithelium. Massive orbital cysts after retinal detachment surgery are rare. The authors review the literature and comment on the differential diagnosis and presumed pathogenesis of this lesion.