A 78-year-old man visited the outpatient clinic with foamy urine and generalized edema that had persisted for 2 months. He had been taking a selective COX-2 inhibitor nonsteroidal anti-inflammatory drug (NSAID) for left osteoarthritis for several months. He had no history of other drug use, recent infections, allergies, or autoimmune diseases. The patient was admitted for further evaluation. His blood pressure was 149/68 mm Hg, heart rate 72 beats/min, height 161.3 cm, and weight 73.7 kg. The laboratory findings were as follows: white blood cell count 4,950/mm3, hemoglobin 12.9 g/dL, platelets 351,000/μL, blood urea nitrogen 29.3 mg/dL, creatinine 1.25 mg/dL, ad total protein 4.2 g/dL. C-reactive protein and uric acid levels were normal. The serum albumin was 2.1 g/dL and total cholesterol 315 mg/dL. The 24-h urinary protein excretion was 6.8 g. The spot urine protein/creatinine ratio was 10.1 mg/g Cr and the urinalysis was negative for occult blood. Serum immunoglobulin (Ig)G, IgA, IgM, complement 3 (C3), complement 4 (C4), and rheumatoid factor levels were normal. Anti-phospholipase A2 receptor (anti-PLA2R) IgG, hepatitis B/C, and HIV serology results were all negative. Serum protein electrophoresis revealed no abnormal monoclonal spikes. Both kidneys were of normal size, and there were no visible cysts or masses on abdominal computed tomography. Because his generalized edema worsened, furosemide was started before the kidney biopsy, and tests were performed to determine the secondary cause of the nephrotic syndrome. The serum CEA was slightly elevated to 4.13 ng/mL and a 2-cm mass was found in the sigmoid colon at colonoscopy. Thereafter, kidney biopsy was performed. The kidney biopsy sampled 12 glomeruli and no changes were observed by light microscopy or immunofluorescence. On electron microscopy, the glomerular basement membrane was moderate to severely irregular in contour, with diffuse effacement of the epithelial foot processes; no electron-dense deposits were found (shown in a–c). The tumor in the sigmoid colon was identified as an adenocarcinoma on histopathological examination, and 5 days later, the patient was also diagnosed with MCD. His generalized edema was well controlled with furosemide, and there was no evidence of metastasis; therefore, surgery was requested. The patient agreed not to take steroids for the nephrotic syndrome and to be followed to assess his improvement after surgery. On the 12th day of admission, laparoscopic anterior resection was performed to remove the tumor. On postoperative day 8, the spot urine protein/creatinine ratio fell significantly to 2.9 mg/g Cr from 10.1 mg/g Cr (shown in ). His foamy urine and edema improved. The patient stopped taking furosemide and was discharged. The patient visited the outpatient clinic on the 14th postoperative day. The spot urine protein/creatinine ratio was 0.1 mg/mg Cr, and the other laboratory abnormalities had normalized.