Chinese herbs nephropathy is characterized by an extensive interstitial fibrosis and by a rapid evolution to end-stage renal failure. We thus decided to try steroid therapy (prednisolone 1 mg/kg for 1 month, tapered off 0.1 mg/kg every 2 weeks) in cases with moderate renal failure and evidence of deterioration in renal function. Our steroid group (SG) consisted of 12 female patients with biopsy-proven renal fibrosis who were followed for at least 12 months after the initiation of steroids. Plasma creatinine level (Pcreat) ranged from 1.8 to 3.9 mg/dL (mean +/- SEM, 2.8 +/- 0.2 mg/dL) when steroids were initiated at t = 0. Renal failure was in progression since Pcreat was 2.1 +/- 0.1 mg/dL (P = 0.022) 3 months before t = 0. Our control group (CG; N = 23) was selected retrospectively from among the 81 patients in the Belgian Register of Chinese Herbs Nephropathy. Compared with the CG, renal function was better preserved in the SG (Pcreat; mean +/- SEM): SG v CG, 2.9 +/- 0.3 mg/dL v 5.3 +/- 0.5 mg/dL at 6 months (P = 0.0024) and 4.0 +/- 0.7 mg/dL v 7.1 +/- 0.5 mg/dL at 1 year (P = 0.001). The slope of the reciprocal serum creatinine concentration was similar in both groups before t = 0 (-0.0463 mg/dL/mo in the SG v -0.0438 mg/dL/mo in the CG; P = 0.83), but it became less steep after initiation of steroid therapy (between 0 and 6 months, -0.000742 mg/dL/mo in the SG v -0.0284 mg/dL/mo in the CG; P < 0.001). Finally, only two of the 12 patients in the SG required dialysis at 1 year compared with 16 of the 23 patients in the CG (P = 0.0045). We conclude that steroid therapy slows the progression of renal failure in a disease characterized by an interstitial fibrosis that progresses quickly despite the fact that the insulting agent has been withdrawn. This supports the hypothesis that renal interstitial fibrosis may be an immune-mediated process.