A 62-year-old female visited our clinic with progressively decreased vision in both eyes beginning 12 years prior. Idiopathic corneal opacity in all layers of the cornea was found in both eyes. One year later, we performed penetrating keratoplasty on the undiagnosed right eye. During post-surgical follow-up, corneal edema and stromal opacity recurred, and penetrating keratoplasty was performed two more times. The patient's total serum protein level, which had previously been normal, was elevated prior to the final surgery. She was diagnosed with monoclonal gammopathy of undetermined significance. We made a final diagnosis of monoclonal gammopathy-associated crystalline keratopathy after corneal biopsy. Monoclonal gammopathy-associated crystalline keratopathy is difficult to diagnose and may lead to severe visual loss. A systemic work-up, including serologic tests like serum protein or cholesterol levels, is needed in patients with unexplainable corneal opacity
