OBJECTIVE: To investigate the role of mast cells in the development of renal interstitial fibrosis in children with Henoch-Schonlein purpura nephritis (HSPN) and possible mechanisms. METHODS: Paraffin-embedded renal biopsy tissue sections from 20 children with HSPN were examined for the levels of tryptase-β and transforming growth factor-β1 (TGF-β1) by immunohistochemical staining. Mast cells were counted by toluidine blue staining. Masson staining was used to assess the level of renal interstitial fibrosis and renal histopathological scores. Normal renal tissue sections from 5 nephrectomized children for nephroma were used as control group. RESULTS: The percentages of positive tryptase-β cells-and mast cells and the TGF-β1 expression in the HSPN group were significantly higher than those in the control group (P <0.05). The percentages of positive tryptase-β cells and mast cells and the TGF-β1 expression in renal tissue were positively correlated with the glomeruli histopathological score (r =0.940,0.920,0.937, respectively; P <0.05) and were also positively correlated with the histopathological score of renal interstitium (r=0.903,0.859,0.948, respectively; P <0.05). The level of renal interstitial fibrosis was positively correlated with the percentages of positive tryptase-β cells and mast cells and the expression of TGF-β1 (r =0.790, 0.766, 0.858, respectively; P <0.05). There was a positive correlation between the percentages of positive tryptase-β cells and mast cells (r =0.941, P <0.05), between the percentage of positive tryptase-β cells and the TGF-β1 expression (r =0.897, P <0.05) and between the percentage of positive mast cells and the TGF-β1 expression (r =0.942, P <0.05). CONCLUSIONS: Tubulointerstitial mast cell infiltration is associated with the development of renal interstitial fibrosis in children with HSPN. Mast cells together with TGF-β1 and mast cell-derived tryptase-β may be involved in the development of the renal interstitial fibrosis in HSPN.［Chin J Contemp Pediatr, 2007, 9 (2):125-128］