Objectives: Cystatin C is emerging as an endogenous marker of glomerular filtration rate. This study sought to assess the usefulness of serum cystatin C as a marker of glomerular filtration rate in comparison with serum creatinine and serum creatinine-based glomerular filtration rate estimations in voluntary kidney donors. 99m Tc-DTPA double plasma sampling method. Glomerular filtration rate-double plasma sampling method was used as a reference value. Results were expressed as means ± SD. Results: The mean age of the participants was 44.23 ± 8.61 years old (19 women, 16 men). The mean serum creatinine was 0.83 ± 0.14 mg/dL, and the mean serum cystatin C was 0.71 ± 0.12 mg/L. Serum cystatin C showed significant correlation with serum creatinine (r = 0.864; P < .001). Glomerular filtration rate-MDRD showed the strongest correlation with glomerular filtration ratedouble plasma sampling method (r = 0.93; P < .001), followed by glomerular filtration rate-CockcroftGault (r = 0.76; P < .001 ), serum creatinine (r = -0.68; P < .001), and serum cystatin C (r = -0.59; P < .001). The mean serum cystatin C values were 22.6% higher in men than in women. There was a significant correlation of serum cystatin C with glomerular filtration rate-Cockcroft-Gault (r = -0.50; P = .002 ), glomerular filtration rate-MDRD (r = -0.59; P < .001 ), and glomerular filtration rate-double plasma sampling method (r = -0.59; P < .001 ). Conclusions: Serum cystatin C is an optimal marker of glomerular filtration rate in voluntary kidney donors.