OBJECTIVE The purpose of this study was to report the occurrence of crystalline keratopathy and of orbital infiltrative disease resulting from crystal-storing histiocytosis (CSH) in a patient with monoclonal gammopathy of undetermined significance. METHODS The authors conducted a review of a medical record and immunohistopathologic studies. RESULTS A 66-year-old man presented with a 3-year history of unilateral progressive ptosis, proptosis, and external ophthalmoplegia. Magnetic resonance imaging showed orbital fat expansion and extraocular muscle thickening with gadolinium enhancement. The patient also had bilateral crystalline keratopathy and had undergone penetrating keratoplasty in one eye. The urine and serum showed elevated levels of immunoglobulin, but the bone marrow aspirate was normal. The systemic evaluation was consistent with monoclonal gammopathy of undetermined significance. An orbital fat biopsy revealed histiocytes engorged with lambda light chain crystals. The corneal stroma also showed positive immunostaining for lambda light chains. The patient was diagnosed with orbital CSH and with lambda light chain crystalline keratopathy. CONCLUSIONS CSH is characterized by the accumulation of reactive histiocytes filled with immunoglobulin crystals in various tissues and is frequently associated with systemic hyperglobulinemic states. For unknown reasons, in this patient, a systemic immunologic disorder led to lambda light chain abnormalities with histiocyte infiltration of the orbit and corneal deposition. Although CSH is rare, it should be part of the differential diagnosis of orbital infiltrative disease with or without crystalline keratopathy.