The incidence of multiple myeloma (MM) is strongly related to age, peaking at 60-70 years. Under 40-year-old, multiple myeloma is rarely diagnosed. Although kidney involvement is important and common (up to 50%) complication of multiple myeloma, myeloma cast nephropathy is rarely reported in young adult. We report a case of 32-year old male who was diagnosed as multiple myeloma with light chain cast nephropathy, presenting as rapidly progressive renal failure. Case: A 32-year-old male patient transferred to outpatient clinic for azotemia and anemia. He complained of nausea and anorexia. BP and PR was 130/80 and 75 respectively. He had no underlying disease history. He had have taken the muscle enhancer and proteins for building muscle for several years. In blood chemistry, serum Creatinine was 20.67mg/dL. He started acute hemodialysis and was admitted in nephrology ward for evaluation for acute kidney injury. In serologic evaluation, C3, C4, FANA, anti-ds DNA antibody, ANCA, and anti-GBM antibody were normal. Free λ light chain was increased in serum and urine (Serum FLC assay: 58200.00, Urine FLC assay: 11130). On kidney biopsy, light chain crystalline (Lambda) cast and inflammatory cell infiltration with eosinophil was observed. Pathologist made a diagnosis of cast nephropathy and acute interstitial nephritis. Bone marrow study was performed. Finally, he was diagnosed with light chain MM (BM-PC 88%) with cast nephropathy. VTD (Bortezomib, Thalidomide, Dexamethasone) induction chemotherapy was done. On 30th admission day, the patient underwent regular hemodialysis and scheduled chemotherapy.