This is the first report from Saudi Arabia studying the normal reference intervals in adult Saudi subjects and evaluating serum cystatin C as a prospective marker for the assessment of the glomerular filtration rate (GFR). Three hundred healthy adult Saudi subjects including 156 males (52%) and 144 females (48%), with a mean age of 31.21 +/- 9.82 years were prospectively studied to establish normal reference ranges for cystatin C. A total of 68.34% of the study patients were in the age-group of 21-40 years. The mean serum cystatin C in the 300 healthy subjects was 0.751 +/- 0.11 mg/L (0.50 - 1.09), increasing gradually with age: it was 0.738 +/- 0.11 mg/L (0.51 - 1.09) in the age-group 21 - 30 years and 0.807 +/- 0.12 (0.51 - 1.09) among subjects who were > 50 years of age. The mean serum cystatin C in females (0.778 +/- 0.118 mg/L) was significantly hig-her than in males (0.726 +/- 0.095 mg/L) (p < 0.0001). The serum cystatin C level was within the defined reference range of 0.53 - 0.95 mg/L in 95% of the subjects with a mean value of 0.74 +/- 0.097 mg/L, and was falling within the 95% confidence interval of 0.73865 - 0.7637 mg/L, and with 98.84% area under the curve (AUC). All the other renal function markers (urea, serum creatinine, calculated GFR, BMI) among the studied subjects were within the normal reference ranges for adult Saudi population. The serum cystatin C level had a significant correlation with the body mass index (BMI) (r = 0.155; p = 0.007) and a correlation with serum creatinine as well (r = 0.009; p = 0.873). It showed a negative correlation with calculated GFR as per Cockroft-Gault equation (r = - 0.101; p = 0.083).