Percutaneous renal biopsy was performed on 5 patients with selective vitamin B<sub>12</sub> malabsorption of whom 3 had proteinuria. Light microscopy showed slight prominence of the mesangial areas but otherwise the findings were normal. Electron microscopy showed increased mesangial matrix, thickening of the basement membrane at the mesangial areas and dark mesangial deposits. Light flocculent subendothelial material, moon craters, membranous convoluted structures, extracellular round particles and occasional intracellular microtubular inclusions were also seen in the glomeruli. The tubular basement membrane appeared thickened around a few tubules, showing membranous vesicular debris and convoluted structures. The capsular membrane also showed vesicular debris. On immunohistochemical examination the glomerular deposits contained immunoglobulins but not complement – an argument against their immunological nature. Deposits were seen only in patients who had been on inadequate treatment for years before the biopsy. Adequate treatment after correct diagnosis decreased the amount of deposits. Proteinuria did not apparently depend on the presence of glomerular deposits.