To determine whether the urinary level of interleukin 6 (IL-6) measured by enzyme-linked immunosorbent assay (ELISA) can be used as a marker of mesangial proliferation, we studied urinary levels of IL-6 in 124 patients with primary and secondary glomerulonephritis, using ELISA. Although urinary levels of IL-6 were correlated with the degree of mesangial proliferation, there was no correlation between urinary levels of IL-6 and urinary protein excretion or renal function. Urinary levels of low-molecular-weight proteins, which are parameters of tubular dysfunction, were not correlated with the urinary excretion of IL-6. These results suggest that the urinary level of IL-6 may be a useful marker for mesangial proliferation.