Two patients with an uncommon form of glomerulonephritis are described. The main clinical features were hematuria and proteinuria associated with normal renal function. The glomerular lesions consisted of mesangial hypercellularity and capillary wall thickening. Immunofluorescence was positive for IgG and C3 in both cases. Widespread deposition of microfibrils (mean diameters 17.0 nm and 18.4 nm) within mesangial areas and capillary basement membranes was seen on electron microscopy. Congo red staining for amyloid was negative. In both patients there was no evidence of underlying disease or extra-glomerular involvement and hence the disorder appeared to represent a primary glomerulonephritis.