The renal biopsy findings in 40 patients with segmental glomerulonephritis are reported. The term is used to describe a condition in which one or more segments of the glomerular tuft is involved by disease when other segments appear unaffected on light microscopy. The word 'focal' is not used as it may be taken to imply that the changes affect some glomeruli but not others and the evidence for this is not convincing. Segmental glomerulonephritis was a relatively common finding in cases of proteinuria with or without the nephrotic syndrome. The severity of the glomerular changes did not correlate with the ultimate prognosis. On the other hand the tubular and interstitial changes, as assessed by a grading procedure and by point counting, were significantly less severe in those patients who showed clinical recovery than in those who did not.