Mast cells (MC) are involved in the pathogenesis of interstitial fibrosis, acute renal transplant rejection, and chronic allograft nephropathy. The aim of this study was to evaluate MC tryptase concentrations in the sera of 58 renal transplant recipients at various times after surgery in relation to graft function. We observed that kidney transplantation patients showed much higher serum tryptase concentrations than healthy controls. We demonstrated a positive correlation between serum tryptase concentration and hemoglobin, hematocrit, red blood cell count, and hepatic cell damage. We were not able to show any direct correlation between serum tryptase concentration and graft function. The clinical relevance of these findings demand further investigation.