Podocytes form an outer aspect of the glomerular capillary wall and play a decisive role in its permeability for macromolecules. The main podocyte surface antigen podocalyxin, a highly electronegative sialoglycoprotein, prevents the podocyte foot processes from collapsing. Podocyte damage in glomerular disease is supposed to be accompanied by podocyte detachment, and shed podocytes and their fragments (marked by podocalyxin) may be identified in the urine. Using anti-podocalyxin monoclonal antibody, PCX+EL were counted by FACS in 38 patients with various types of active glomerulonephritis, 15 patients with chronic glomerulonephritis in long-term remission and 44 healthy controls. Urinary levels of PCX+EL were significantly higher in patients with active glomerulonephritis compared to patients with chronic glomerulonephritis in longterm remission (93 +/- 100 vs. 6.3 +/- 3.2/microl of urine, P < 0.000001) and healthy controls (4.4 +/- 2.6/microl of urine, P < 0.000001 compared to active glomerulonephritis, n.s. compared to chronic glomerulonephritis in longterm remission). These preliminary data suggest the potential of this simple method to monitor the activity of glomerular disease. Further prospective studies of larger cohorts of patients with individual glomerular diseases are clearly warranted.