Corneal intraepithelial neoplasia (CIN) is a premalignant lesion of the corneal epithelium and limbus. In this condition, the severity of the dysplastic cells is determined by pathology and classified as mild, moderate, or severe. Severe dysplasia has dysplastic cells spanning the epithelial layer but not yet invading the basement membrane and is called carcinoma in situ. The etiology of the dysplastic cellular growth is thought to be actinic damage. The changes from CIN do not cause pain but will reduce vision once the changes invade the visual axis. CIN tends to affect older patients with sun exposure. Patients with CIN may complain of reduced vision, and vision must be checked.