Lobular glomerulonephritis is an entity first thought to have represented a primary disease of uncertain histogenesis, but more recently has generally been considered to represent a morphologic variant of membranoproliferative glomerulonephritis. We have encountered five patients who were found to have a lobular glomerulonephritis by renal biopsy, but in whom features of membranoproliferative glomerulonephritis types I, II, or III, could not be demonstrated and in whom alternate known diagnostic categories could be excluded. We suggest that lobular glomerulonephritis, or alternately, idiopathic nodular mesangial sclerosis, is an uncommon but persistent disease entity with a distinctive pathologic appearance and unknown pathogenesis.