This paper gives an account of five patients with glomerulonephritis complicating subacute infective endocarditis. In three patients with focal (segmental) glomerulonephritis granular deposits of immunoglobulin and C3 and reduced serum complement suggested an immune-complex aetiology; but in two patients with a diffuse proliferative glomerulonephritis no Ig or C3 was detected in the diseased glomeruli and the pathogenetic mechanism remained undetermined. Four of the five patients developed renal failure needing dialysis.