OBJECTIVES: To validate a scoring system to assess the severity of renal lesions and to correlate histology with clinical findings. We also examined the efficacy of treatment with prednisone (1 to 2 mg/kg/d) and azathioprine (1 to 2 mg/kg/d) for severe Henoch-Schonlein purpura (HSP) nephritis. METHODS: Twenty patients were evaluated retrospectively. All underwent biopsy before treatment, and 13 underwent biopsy after therapy. We developed a scale based on glomerular, tubulointerstitial (TI), and vascular changes and assigned all specimens acuity, chronicity, and TI scores. The outcomes of 17 patients were compared with those of a historical control group. RESULTS: Chronicity score at initial biopsy increased with increasing delay between onset of renal involvement and first biopsy (rho = 0.55, P =.016) but did not progress after treatment was initiated. Both acuity (rho = 0.57,P =. 016) and TI (rho = 0.69, P =.003) scores correlated with clinical severity at first biopsy. The TI score correlated negatively with serum albumin (rho = -.60, P <.01). Significantly more patients in the study group than in the control group had a favorable outcome (15 [88%] of 17 vs 32 [54%] of 59, P =.011). CONCLUSIONS: Our scale reflects disease activity and highlights the importance of TI changes in severe HSP nephritis. Outcome comparisons indicate that early treatment with prednisone and azathioprine prevents progression of chronic changes and improves outcome.