Abstract Transforming growth factor-β1 (TGF-β1) has an important role in the pathogenesis of glomerular damage by influencing matrix metabolism. An association of TGF-β1 with glomerulosclerosis and interstitial fibrosis has been shown in various renal diseases, suggesting that TGF-β1 may serve as a diagnostic marker of glomerular diseases. The aim of this study is to determine the usefulness of urinary TGF-β1 values to monitor therapeutic effects of steroids in patients with immunoglobulin A (IgA) nephropathy. Concentrations and activation rates of TGF-β1 (mature/total) were determined in urine of patients with renal diseases by means of a double-antibody enzyme immunoassay. The urinary TGF-β1 level before steroid therapy was compared with renal histological characteristics, creatinine clearance, and proteinuria in patients with a variety of renal diseases. Urinary excretion of total and mature TGF-β1 was significantly greater in patients with crescentic glomerulonephritis and IgA nephropathy than in healthy controls, whereas the activation rate of urinary TGF-β1 was similar among patients with other renal diseases. Urinary TGF-β1 excretion at the time of renal biopsy significantly correlated with the degree of crescent formation in patients with IgA nephropathy, but not in those with glomerular sclerosis or tubulointerstitial fibrosis. Urinary excretion of total and mature TGF-β1 was reduced in patients with IgA nephropathy after treatment with prednisolone (0.8 mg/kg/d) for 1 month. The activation rate of urinary TGF-β1 also decreased significantly after steroid therapy. Urinary TGF-β1 values therefore may be useful to assess disease activity or the effects of steroid therapy in patients with IgA nephropathy. © 2001 by the National Kidney Foundation, Inc.