The level of plasma fibronectin (FN) is increasing in prognostically unfavourable morphological forms of childrens glomerulonephritis being in direct relationship with the activity index and sclerosis index. Urine FN excretion depends upon generalization of the tubulointerstitial changes, presence in the kidney of synechia and segmentary sclerosis of the glomeruli capillaries. A direct correlation is established between FN in the renal tissue and the degree of the mesangial cell proliferation and the spread of sclerotic changes.