Proteinuria is characteristic of many glomerular conditions, and often exceeds 2-3 g/24 h. There are several possible routes by which such profuse proteinuria might contribute to progression of the underlying pathology, whatever its type. First, proteinuria leads to a transit of protein through glomerular structures, including the glomerular capillary basement membrane, the mesangium and the epithelial cells, and to increased traffic of protein through the proximal tubules by pinocytosis of filtered protein. This traffic may be toxic to the cells concerned, and there is some evidence from 'overload' proteinuria induced in animals that this is so. Second, proteinuria leads to secondary hyperlipidemia with raised lipoproteins: mesangial cells have receptors for lipoproteins and in vitro, they are damaged by high concentrations, and there is evidence that hyperlipidemia leads to glomerulosclerosis. Third, proteinuria leads to hyperaggregability of platelets through alterations in plasma proteins, principally a fall in concentration of serum albumin and a rise in that of the von Willebrand factor, and possibly to increases in humoral coagulation cascades as well through losses of regulator proteins such as antithrombin III. There is evidence that anticoagulation and antiplatelet drugs will inhibit glomerulosclerosis in animals. Whether all or any of these mechanisms operate in human disease is not known; however, prognosis correlates well with duration and intensity of proteinuria in almost all proteinuric states and with the appearance and persistence of proteinuria in hematuric conditions. Therapies designed to reduce proteinuria per se may have a role in the treatment of glomerulopathies.