Recurrent lobular glomerulonephritis in an allotransplanted kidney was studied by light microscopy, immunofluorescent techniques and electron microscopy. Three successive biopsies of the graft and the nephrectomy of the transplant were performed. The histological and clinical evolution of the lobular glomerulonephritis of the graft paralleled that of the lobular glomerulonephritis of the donor kidney. The clinical evolution of the recurrent lobular glomerulonephritis in the grafted kidney of the recipient was comparable to that of the original lobular glomerulonephritis, of the recipient's own kidney. Serial biopsies showed that immunoglobulin deposits appeared prior to cellular proliferation. This observation is consistent with the hypothesis that immune complexes have a primary role in the pathogenesis of human glomerulonephritis.