Rationale:Corneal opacity can be caused by various disease. Generally, the opacity gradually increases as the disease progresses. Sudden corneal opacity is mainly caused by corneal trauma, toxic drugs entering the cornea, or acute edema of the keratoconus. However, sudden corneal opacity caused by diabetes has not been reported.Patient concerns:A 60-year-old man reported blurred vision and the black eye became white in appearance in the left eye for 5 days. The patient had a history of diabetes which had not been treated.Diagnoses:He underwent slit-lamp examination, anterior segment optical coherence tomography, ultrasound bio microscopy, B-mode ultrasound, corneal endothelial examination, random blood glucose testing, and other examinations. The diagnosis of Diabetic Keratopathy was made.Interventions:Topical glucocorticoids and dilating eye drops were administered and undergo blood sugar control treatment.Outcomes:The corneal of the patient was completely transparent in a few days, and the flocculent exudation in the anterior chamber disappeared.Lessons:Although diabetes generally causes chronic corneal edema, acute corneal edema may also occur when blood sugar is poorly controlled. Therefore, when we see sudden corneal opacity without obvious incentives, we must consider systemic diseases, especially diabetes.