A healthy 43-year-old officer of a merchant ship at sea developed pain, redness, and photophobia in his right eye. During the next 2 weeks, he noted the presence of a band of opacity spreading from his temporal limbus toward his central cornea. His episcleral vessels were engorged in a distribution contiguous with the peripheral, sectorial, flecklike corneal opacities. The opacity had progressed during topical and systemic antibiotic therapy, but halted with use of topical corticosteroids. Systemic evaluation showed mild IgM monoclonal gammopathy. Transmission electron microscopy of a corneal biopsy specimen revealed electrondense fibrils identified as immunoprotein. To our knowledge, this is the first report of a case of acute unilateral deposition of corneal immunoprotein in a patient with monoclonal gammopathy. Clinicians should begin with a broad differential diagnosis when evaluating patients with corneal opacity.