The glomerular basement membrane (GBM) is the vital barrier between blood and glomerular filtrate. Diseases that damage it generally cause haematuria initially, but as architecture becomes disordered proteinuria becomes an increasing feature. Inflammatory glomerulonephritis most commonly does this, but genetic abnormalities of GBM components are another important mechanism. Alport's syndrome, a hereditary nephritis associated with deafness, is the second commonest genetic cause of renal failure. Thin basement membrane nephropathy is thought to be the underlying diagnosis in 25% of patients presenting to nephrologists with microscopic haematuria.