Background: The development of early renal interstitial fibrosis (IF) in renal allografts is likely to depend on multiple factors. We studied retrospectively renal biopsies from cadaveric human renal allografts, transplanted from 1996 to 1998, with the intention of detecting early fibrotic changes and determining the underlying cellular mediators. We studied 23 transplant patients whose 46 renal biopsies were analysed, including a donor biopsy taken routinely at implantation from each patient and 23 follow-up biopsies, taken as clinically indicated over a period of 3 months following transplantation. Methods: Histological evaluation of induction and progression of fibrosis relied on point count analysis of conventional (Masson’s trichrome/MT) and immunohistochemical staining for collagen III and IV, and alpha-smooth muscle actin (α-SMA) as a marker of myofibroblast differentiation. Mast cells (MC) were counted in sections stained with an anti-human mast cells tryptase monoclonal antibody. Activated macrophages as well as total, helper and cytotoxic T-lymphocytes were identified on frozen sections by direct immunofluorescence using mouse anti-CD71, CD3, CD4 and CD8 antibodies respectively. Eosinophils (E) were counted in hematoxylin and eosin (HE)-stained sections. Changes in interstitial fibrosis (IF) scores were evaluated and correlated with myofibroblasts, MC, E and lympho-monocytic cells. Results: We noted a significant increase in IF over a 3 months period following transplantation. There was also a significant increase in α-SMA<sup>+</sup> cells, MC and E counts from implantation to follow-up biopsies. Similarly, there was a significant increase in interstitial infiltration by T-lymphocytes (modal category = 2 versus 0, p = 0.012) but not by macrophages. MC at implantation and follow-up were found to be predictive of IF (immunostainable collagen III) at follow-up (R<sup>2</sup> = 0.510, p = 0.023 and p = 0.030). Further, the predictive value for total T-lymphocyte infiltration at follow-up was also significant (R<sup>2</sup> = 0.617, p = 0.036). A strong correlation was found between α-SMA<sup>+</sup> cells and MC counts at implantation (r = 0.7259, p < 0.001) and in follow-up biopsies (r = 0.5183, p < 0.01). However, there was no correlation between E counts and either α-SMA<sup>+</sup> cells or MC either at implantation or in follow-up biopsies. Based on changes in interstitial immunostainable α-SMA, our patients were divided arbitrarily into 2 groups; group 1 (n = 12) with >100% increase in α-SMA and group 2 (n = 11) with <100% increase. Group 1 patients differed significantly from group 2 regarding the degree of MC infiltration at follow-up (t = 0.4519, p < 0.05) with the mean increase in MC count from implantation to follow-up biopsies being +5.1 cells/high power field (HPF) in group 1 and +0.8 cell/HPF in group 2 (p = 0.0237). MC counts in group 1 were associated with a higher modal category (greater than one) of cytotoxic: helper T-lymphocytes ratio compared to group 2 (2:1 versus 1:1 respectively). Conclusion: Multiple cells may contribute to early interstitial fibrosis in a subgroup of human renal allograft recipients with MC playing a major role.