Clinical and pathologic findings are described in 14 patients whose main abnormality was an excessively thin glomerular basement membrane (GBM). The subjects were aged 11-51 years, the majority males: most were referred because of hematuria, but proteinuria was the main problem in 2, while hypertension or renal functional impairment was found in several, and 1 was in end-stage renal failure. A history of apparently similar renal symptoms was obtained in another 3 family members. In addition to GBM abnormalities, renal biopsy features included a slight mesangial matrix increase, occasional mesangial cell excess and often appreciable pedicel effacement. There were scanty electron-dense deposits. The mean thickness of the GBM varied from 206 to 301 nm, whereas in IgA nephropathy patients used as controls it was 356-464 nm. It is concluded that some of the lesions in adults are 'benign', and some progressive. 'Thin membrane nephropathy' is comparatively common among patients seen by the authors, and it is suggested that awareness of this condition will result more often in its recognition.