We describe a distinctive and previously undescribed abnormality of the kidney. It consists of a combination of changes in the glomerulus and in the proximal convoluted tubule. The glomerular abnormality consists of a well-localized collection of intracapillary foam cells and marked vacuolation of the adjacent glomerular epithelial cells. The abnormality is always situated in the same position in the glomerular tuft namely adjacent to the origin of the proximal convoluted tubule, with adhesion to Bowman's capsule. The rest of the tuft appears normal by light microscopy but shows foot-process fusion by electron microscopy. The abnormality of the proximal convoluted tubular cells invariably affects the first part of the tubule adjacent to the glomerulus but in some cases involves the tubules more extensively in the cortex. Study of a series of 100 2-micron serial sections from one case indicated that probably every glomerulus had a lesion at the origin of the tubule. A study of 185 renal biopsies nearly all with segmental lesions revealed biopsies from 20 patients with this distinctive abnormality. There were 14 males, age 20-57, median 44 years, and six females age 19-65, median 22 years, all presenting with proteinuria, nearly all with the nephrotic syndrome. All except four were treated with steroids and in all those treated the proteinuria improved. Only one patient not treated with steroids had progressive renal impairment. Four patients died, none from renal failure. This lesion, the glomerular tip lesion, as we have called it, seems to be a well-defined and specific pathological entity. It has some similarities to minimal change nephropathy but there are clear and important differences. Previously it may have been included in series of cases of 'focal glomerulosclerosis' but that term is imprecise and is generally taken to have an unfavourable clinical course. For these reasons it is inappropriate to use 'focal glomerulosclerosis' as a name for the glomerular tip lesion.