Purpose: To report a case with focal corneal decompensation after filtering surgery with inadvertent inadequate irrigation of mitomycin C (MMC). Methods: Case report and review of literature. Results: A 25-year-old man first referred with the complaint of photophobia. His ocular examinations revealed diffuse keratic precipitates and many iris nodules in both eyes. The primary diagnosis was idiopathic bilateral granulomatous anterior uveitis. The intraocular pressure (IOP) gradually increased in the left eye and was not controlled with a prescription of topical antiglaucoma medications. As the disease progressed, the left eye underwent filtering surgery with MMC 0.02%. The postoperative period was uneventful, and the anterior chamber was deep after surgery. The IOP was controlled without medications; however; the inferior third of the cornea was edematous because of severe endothelial dysfunction. Conclusions: Inadequate irrigation of MMC during filtering surgery can cause focal corneal decompensation.