Oedema is a common clinical manifestation of nephrotic syndrome. However, the pathophysiological mechanism of sodium retention in nephrotic syndrome has been intensely debated for decades. Several clinical and experimental observations argue against the classic or "underfill" hypothesis of oedema formation in nephrotic syndrome. In many patients, oedema formation in nephrotic syndrome is due to the kidney being intrinsically unable to excrete salt and is unrelated to systemic factors (i.e. hypoalbuminaemia, decreased “effective” arterial blood volume, and secondary hyperaldosteronism). The cortical collecting duct is the nephron site of sodium retention in nephrotic syndrome. Activation of the epithelial sodium channel in the cortical collecting duct is responsible for sodium retention in nephrotic syndrome. In nephrotic syndrome, a defective glomerular filtration barrier allows the passage of proteolytic enzymes or their precursors, which have the ability to activate the epithelial sodium channel, thereby causing the the subsequent sodium retention and oedema.