Macrophage migration inhibitory factor (MIF) is a pro-inflammatory cytokine that is a potent activator of macrophages and T cells. Previous studies have shown that local MIF production is increased in acute renal allograft rejection, suggesting that it may play an important role in the rejection process. Thus we assessed the ratio of MIF to creatinine in urine in two pediatric patients with acute rejection episode. Case one was a 10 years old girl with hypodysplasia who underwent transplantation 3 years ago. Urine MIF/cr ratio was 1.5 pg/micromole cr in outpatient visit. She involved by acute rejection episode. In this phase the urine MIF/cr ratio was 43.3. The second case was a 8 years old girl with cystinosis who underwent transplantation 1 mo ago. Her assessment showed that the urine MIF/cr was 96.3 in phase of acute rejection. We measured urine MIF/cr in 4 patients with renal transplantation who were not in acute rejection and mean age, sex and year of transplantation of them was similar to our patients. The mean urine MIF/cr ration in them was 2.5 pgr/micromole cr. We also measured the urine MIF/cr ratio in 4 healthy children as control group and this ratio was 1.9 in these children. This case study demonstrates that the concentration of MIF in urine increases with episodes of acute rejection. This is important for diagnosis of acute rejection and differentiation of it from other states such as cyclosporine nephrotoxicity.