Arachnoid cysts are benign intracranial lesions that are typically diagnosed incidentally. We describe the case of a 56-year-old man who presented with a multiloculated arachnoid cyst of the middle cranial fossa that extended into the sphenoid sinus. The lesion was identified on computed tomography of the head, which had been obtained for an unrelated investigation. However, establishing a definitive diagnosis proved to be difficult. Because the cyst had caused extensive skull base erosion, the patient was managed conservatively with close observation. We report the radiographic progression of this lesion during more than a decade of follow-up, and we review the literature pertaining to the presentation, pathophysiology, and treatment of arachnoid cysts.