A 23-year-old male whose uncle died of nephronophthisis, and whose pathology is also discussed, presented with 5 g of protein in a 24-hour urine collection. Nephrogenic diabetes insipidus and salt wasting were present in addition to azotemia. Characterization of the proteinuria, including elevated alpha globulins by electrophoresis and markedly elevated urinary beta-microglobulins by radioimmunoassay (49.55 mg/L) indicated predominantly tubular proteinuria. A percutaneous renal biopsy showed normal glomeruli, interstitial inflammation and fibrosis, and tubular atrophy. Electron microscopy revealed notable alterations of the tubular basement membrane.