Purpose. To describe the clinical findings of a patient who initially underwent surgery for a pterygium and who was finally diagnosed with invasive squamous cell carcinoma. and to determine the necessity and importance of pathologic investigation in all cases of a presumed pterygium. Methods. A case report. Results. A 63-year-old man was referred for severe pain, redness. and purulent secretion in his right eye. A year earlier. he had under-one excision of a pterygium in his right eye by a bare sclera technique without any adjunctive antimetabolite in a private eye clinic. Furthermore, 6 months after this operation, he had undergone autologous conjunctival graft surgery because of dehiscence of the Surgical site. His visual acuity was Only light perception in the right eye. Slitlamp biomicroscopy showed severe nasal corneoscleral melting, purulent secretion, conjunctival hyperemia, corneal edema. hypopyon, intracamaral hemorrhage. and lid swelling. Ultrasonography showed a right hypoechoic mass invading the nasal part of the globe and a totally detached retina. Orbital magnetic resonance imaging showed a large (2 cm in diameter), irregular, lobulated mass invading the globe in the medial part of the right orbit. Results of a biopsy were consistent with squamous cell carcinoma. Because of the extensive intraocular involvement at the time of the diagnosis, subtotal orbital exenteration was performed. Conclusions. All pterygia Should be evaluated meticulously with regard to possible underlying causes, Such as carcinoma in situ or squamous Cell carcinoma, and all excised lesions should also be evaluated pathologically to prevent Such serious adverse Outcomes that are mentioned in this case report.