A 68-year-old male patient visited the hospital for the management of bilateral lower extremities edema. He had a 20-year history of diabetes mellitus, complicated by diabetic retinopathy. Laboratory investigations revealed a serum creatinine level of 0.98 mg/dL, hemoglobin 6.6 g/dL, total protein 6.4 g/dL, albumin 2.9 g/dL, and globulin 3.5 g/dL. Urinalysis indicated significant proteinuria with 3+ protein with hematuria and a urine protein-creatinine ratio of 2,221.59 mg/g. After admission, his renal function slowly declines to serum creatinine of 1.33 mg/dL and worsens bilateral lower extremities edema. To address other possible causes of anemia and proteinuria, we evaluated the serum and urine protein electrophoresis with immunofixation and identified a monoclonal lambda subtype of immunoglobulin M (IgM). Further evaluation included a bone marrow biopsy, establishing a hematological lymphoplasmacytic lymphoma (LPL) diagnosis (Fig. 1). The renal biopsy revealed light chain cast nephropathy alongside diabetic nephropathy. Congo red staining yielded negative results, ruling out amyloidosis (Fig. 2). Consequently, a diagnosis of cast nephropathy in the context of LPL was confirmed. We treated with six cycles of bendamustine (70 mg/m²) in combination with rituximab (375 mg/m²), and the patient is currently complete remission state.