Spontaneous crystallization of monoclonal immunoglobulins (crystalglobulin) is a rare complication of multiple myeloma. We describe a 64-year-old Japanese man with skin ulcers and renal failure associated with immunoglobulin G kappa multiple myeloma. Crystallized immunoglobulin was detected in his serum at room temperature. Analysis of the patient's crystalglobulin by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and mass spectrometry suggested that the crystallization was due to abnormal glycosylation of the immunoglobulin light chain. Treatment with thalidomide and dexamethasone improved the severe skin ulcers on the patient's extremities and partially reversed his renal failure. This report is the first of abnormal glycosylation of immunoglobulin possibly caused by modification of N-glycans in the light chain. We concluded that abnormal glycosylation of the immunoglobulin light chain might be the cause of the patient's skin ulcers and renal dysfunction.