Mesangial proliferative glomerulonephritis (mesPGN) is histologically characterized by proliferation of mesangial cells (MC), suggesting the involvement of a growth factor for MC in the pathogenesis of mesPGN. We have previously shown that interleukin-6 (IL-6) induces proliferation of cultured rat mesangial cells, and urine samples from patients with IgA nephropathy contain high level of IL-6 activity. We have also demonstrated that transgenic mice carrying a human IL-6 genomic gene showed severe mesangial proliferation and matrix enlargement. Urinary samples of patients with lupus nephritis as well as IgA nephropathy contain significant IL-6 activity. Over a ten month follow-up, a positive correlation between the urinary IL-6 and pathological score was observed. Hence, measurement of urinary IL-6 can be used as a good indicator for monitoring IgA nephropathy and lupus nephritis. Using RT-PCR methods, IL-6 mRNA was detected in the glomeruli of renal biopsy specimens obtained from patients with IgA nephropathy and lupus nephritis.