Creatinine is the traditional and most commonly used marker of the GFR. It has many practical and theoretical disadvantages.Cystatin C is a promising endogenous marker of GFR, and it might be of particular interest in patients with slightly decreased kidney function .The aim of the study was to evaluate serum cystatin C as a marker of the GFR in patients with various degree of renal diseases in comparison with serum creatinine the most commonly used marker of the GFR.40 subjects were included in this study and were divided into control group comprised 10 healthy subjects with normal renal functions., patients with renal impairment group comprised 20 patients with various degree of renal diseases under conservative mangment, and end stage renal disease group comprised 10 patient suffering from C.R.F on regular haemodialysis. The three groups were subjected to full clinical history, thorough clinical examination, routine investigations to confirm the etiology of renal diseases, kidney function tests (serum urea and creatinine levels), isotop renogram using 99mTc-DTPA-clearance (for measuring GFR) and estimation of serum cystatin C level by particle-enhanced turbidimetric assay. The collected data were statistically analysed and the following results have been obtained:•The levels of serum creatinine and serum cystatin C were significantly lower among control group than those of the patients, while the levels of GFR were significantly higher among control group than those of patients.•There was highly significant negative correlation of GFR with serum creatinine and serum cystatin C but GFR correlated with cystatin C (r = -0.86) more than creatinine (r = -0.75).•There was highly significant positive correlation between serum creatinine and serum cystatin C.•There was highly significant positive correlation between serum creatinine and body mass index and no significant correlation between serum cystatin C and body mass index of patients.•Serum cystain C was more sensitive, specific and accurate than serum creatinine as a marker of GFR. Also, serum cystatin C had higher positive and negative predictive values than serum creatinine.•Serum cystatin C detected early reduction in GFR because the GFR value (90 ml/min/ 1.73 m2) associated with beginning of rise of serum cystatin C above the mean value was higher than the GFR value (76.7 ml/min/ 1.73 m2) associated with beginning of rise of serum creatinine above the mean values.In conclusion, this study demonstrates that serum cystatin C offers a more efficient diagnostic tool than serum creatinine in patients with various degree of renal diseases. Also, cystatin C may be useful for the identifcation of mild impaired GFR without need to correct for age dependency as has to be done with serum creatinine.