The potential effects of macrophage migration inhibitory factor (MIF) on the natural immune response are due to the inhibition of immune cell activation, which is regulated by glucocorticoids. In this study, we investigated MIF -173G/C genotype and C allele frequency in 214 patients with idiopathic nephrotic syndrome (INS) and 103 healthy volunteers. We found significant increases in GC genotype (OR=3, p=0.0009) and C allele frequency (OR=2.5, p=0.0007) in INS. Upon classifying patients as steroid responsive (n=137) or resistant (n=77), a 20-fold over-expression of the CC-genotype was found in the steroid-resistant group (OR=20, p=0.0002). Moreover, a significant increase in C allele frequency in patients with focal segmental glomerulosclerosis (FSGS) has also been noted when compared with other histopathological groups (OR=3.2, p=0.0017). Furthermore, significant increases in the CC genotype (15.6% vs 3.3%) and C allele (75% vs 32%) frequencies have been found in patients with permanent renal function failure (p=0.013 and p=0.0002, respectively). Patients with the CC genotype were found to be at considerably increased risk of permanent renal failure (OR=5.43, p=0.013) and end-stage renal disease (OR=5.53, p=0.020). Additionally, there was a correlation between age of detection of proteinuria and CC genotype. We found an earlier age of onset of proteinuria in patients with the CC genotype (1.9 +/- 1.7 years) than in patients who were GC-heterozygous (3.7 +/- 3.1 years) and GG-homozygous (3.6 +/- 2.9 years, p=0.88). In summary, our results indicate that the MIF -173 C allele confers an increased risk of susceptibility to INS and plays a crucial role in glucocorticoid responsiveness.