Electron microscopic studies of glomeruli and determination of protein clearances were made in seven patients presenting with postural proteinuria, with no evidence of established renal disease. In all cases renal biopsies were taken while the patient had proteinuria; in three another biopsy was obtained in the absence of proteinuria. Individual protein clearances of five serum proteins of widely differing molecular weights were estimated by immunological methods. On electron microscopy focal abnormalities were seen in all glomeruli. ‘Ballooning’ of epithelial cell cytoplasm with patchy fusion of foot processes and increased numbers of Folli bodies were noted. The basement membrane appeared normal in all cases embedded in Araldite. The changes described in epithelial cells are common to most conditions giving rise to proteinuria. There was no significant ultrastructural difference between biopsies taken in the presence of and in the absence of proteinuria. Results of protein clearance studies show that the proteinuria is highly unselective in these patients, and is similar to the excretion pattern found in normal individuals, contrasting with proteinuria in minimal lesion glomerulonephritis which is highly selective. This suggests that in spite of some morphological similarity to minimal lesion glomerulonephritis, postural proteinuria is in fact unrelated to this condition and may be a physiological variant of normal function. This view cannot be established with certainty without long-term follow up and serial examinations of biopsy material from such patients.