1. IntroductionMonoclonal gammopathy is associated with various types of renal injuries, such as light chain cast nephropathy (LCPT), AL amyloidosis, and monoclonal immunoglobulin deposition disease, recently recognized as monoclonal gammopathy of renal significance (MGRS). LCPT is a rare type of MGRS, with approximately only 150–200 cases reported in the literature [1–6]. In LCPT, monoclonal light chains secreted by abnormal plasma cells accumulate in the proximal tubular cells and cause proximal tubular dysfunction, which is clinically characterized by tubular acidosis, normoglycemic glycosuria, aminoaciduria, and hypophosphatemia, collectively called Fanconi syndrome [7].Diagnosis of LCPT is confirmed by findings of renal biopsy and the presence of specific histological features, including cytoplasmic monoclonal light chain inclusions and an increased number of lysosomes in the proximal tubular cells, which are sometimes only detectable by electron microscopy [8]. In addition, the coexistence of LCPT and other paraprotein-related kidney disease has been reported [9–11], which makes the diagnosis more difficult. LCPT often presents as a slowly progressive renal impairment; however, some patients develop end-stage kidney disease or aggressive multiple myeloma [7]. Although the treatment strategy for MGRS has not yet been established due to the rarity and lack of familiarity of this entity, several studies have shown that improvement in renal function can be achieved with hematologic response to chemotherapy, commonly bortezomib for plasma cell dyscrasia, and rituximab for B cell lymphoproliferative disease. Some case reports and case series have shown that chemotherapy directed at multiple myeloma is also effective for LCPT [1–4], but only a few case reports have described its clinical course, and the optimal treatment strategy remains unknown.Herein, we describe the case of a patient with LCPT and severe kidney injury who received bortezomib-based chemotherapy and autologous stem cell transplantation (ASCT). This case report illustrates new insight into the optimal treatment strategy for LCPT in the future.